Loss of Power

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  #1  
Old 11-06-03, 06:05 PM
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Loss of Power

Okay Joe and you other GM experts, give the_tow_guy a little help. Been struggling with a problem on our '99 Isuzu NPR-HD (350 Chevy motor) for a couple of weeks. Unfortunately (!) our troubleshooting has been very off and on because we've been so busy (the annual influx of refugees from the frozen north has begun).

Here's the symptom (this is second-hand because its my partner's truck): Loss of power/bogs down at about 45 mph, especially noticable in instances like car on hook and going uphill over a bridge. Doesn't occur ALL the time, but frequent enough so as to SAY all the time.

No check engine light.

Items eliminated by troubleshooting or replacement:

Fuel pump
Fuel filter
Ignition coil and module
Exhaust system
Plugs/wires/cap

Fuel pump was found to have a bad connector that was causing a voltage drop; apparently unrelated as it didn't fix the original problem. Fuel filter changed as a precautionary measure. Ignition coil and module swapped out with known-good units. Exhaust system is clear; really thought that might have been the culprit. Plugs/wires/cap new enough to preclude any suspicion.

Probably forgetting a couple of things that we've checked and as yet have not found time to get it on a diagnostic machine. Good news is MY truck is running fine, LOL, so HE has to run the flatbed in between running down this head-scratcher.

Any and all ideas greatfully accepted.

TG
 

Last edited by the_tow_guy; 12-23-03 at 12:30 PM.
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  #2  
Old 11-06-03, 08:02 PM
jimmymc
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A few things come to mind...all related to fuel starvation.

Another fuel filter someplace

Screen in the tank...fuel tank may be contaminated

Tank vent

Worn/bent fuel pump rod-(mechanical pump)

Foreign object in tank

Pinched fuel line

Good luck,

jimmymc
 
  #3  
Old 11-07-03, 12:15 AM
mike from nj
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tow-guy, we actually sell these in work. although we see about 200 4-cyl diesels to the one v8 gas. if you need any factory reference material, we have it.

if you have any kind of scan tool, or even a voltmeter with long probes, i would back probe the oxygen sensors and watch what they read when the problem is occuring.

it should be above 750-800 millivolts in power enrichment mode(foot almost on the floor), if it's steady up there, you have a rich exhaust stream and NO fuel problem(you don't even need a fuel pressure gauge with that test)

if it drops down real low, like 100-300 millivolts (.1-.3 volts) then you have a very lean condition and a possible fuel starvation problem, or a weak sensor, the fuel pressure reading while it's happening is the deciding factor

you might have an oxygen sensor starting to fail, but test first!

(gm O2 sensors are almost known to do what you're describing, i've had many isuzu suv's do exactly that)

let us know
 
  #4  
Old 11-07-03, 04:01 AM
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Will it do it cold or hot? Both? A lot of GM modules give up hope when things get hot and it will cause it to flat out/hesitate like that.

I do agree with fuel starvation. I don't have any bulletins for it here, but I do have the factory material at work, so as Mike suggested one of us can send it to you.

Chevy and Isuzu both market a version of that here.
 
  #5  
Old 11-07-03, 04:56 AM
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Near as we can tell, and again this is second hand since I'm not the primary troublehooter, it does not appear to be a fuel DELIVERY problem. Good pump, good pressure, good filter, no obstructions.

Replacement module is something like $125, so we're more than a little reluctant to replace it. What we DID do was swap it out with a unit (module and coil mounted on common bracket) that our favorite garage uses for troubleshooting. Of course, it's only an ASSUMPTION that its not bad also, but there was apparently no change in the symptoms. We DID replace the coil. There may be some difference between hot and cold symptoms, but I think he can induce the problem right from the first startup in the morning.

O2 sensors, there are 4 of them, are on the suspect list, but so far we've not gotten a check engine light. He did pull them all for a visual inspection and said "1" looked a little funky (can't remember the exact description), so was replaced. May have been some improvement, but not significantly so (I've only driven it once in its problem mode, so can't give a real good analysis). As with the module, we are a LITTLE reluctant to just throw sensors at it (@$65/ea), but the symptoms are somewhat similar to what happens on my W-4 when the O2 sensor (only has one)craps out. Biggest difference is that on the W-4 the bogging down is pretty much the entire throttle range and not just under load at moderate speeds. Plus the gas milage goes right down the toilet, and I mean like about half. Not sure if there's been a significant change in the mpg on it or not; will have to ask him.

All things considered, Mike, unfortunately with four sensors it may be easier and more expedious to just change out the other three rather than mess with the voltage checks, but we'll certainly add that to the growing list of things to look into.

Biggest headache? No check engine light to give us a clue!
 
  #6  
Old 11-07-03, 07:02 AM
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TG (you know I don't even know your real name after all these years..lol):

Follow me, old champ. You have the tools to do the job. Have your favorite garage hook this sling shot machine up to a scanner. Let's see some voltage readings from the scanner. This will tell us if we have problems with things before throwing money at the problem with parts (I agree here).

Sounds like you have a good rapport with a good shop with a scanner. Let's go this route first. Hang in there, we'll figure this out...
 
  #7  
Old 11-07-03, 03:19 PM
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I misspoke; biggest headache is getting scanner time with favorite garage. Good garage, stays busy, and will be doing it free so don't want to pester them TOO much. Should be able to get it on within the next few days (we hope). Trying REAL hard not to be shotgunning the troubleshooting. While waiting for the scanner time, trying to address bits and pieces that we can check without scanner or $ expenditure. Forgot (AGAIN!) to ask him for gas milage info.

SIR or MISTER Tow Guy will be sufficient. LOL

John aka "Top"
 
  #8  
Old 11-07-03, 04:21 PM
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Towguy while you are getting it scanned have them check CMP retard if possible if its more than + or -2 degrees the distributor will need moved(turned)to correct the problem.This will sound like I'm nuts but pull the dist and see if the gear is worn.
 
  #9  
Old 11-07-03, 10:16 PM
mike from nj
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the sensors that help trim the fuel 'curve' are before the cat, the ones after it only monitor cat efficiency.
 
  #10  
Old 11-08-03, 05:28 AM
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Thanks for the additional ideas, guys.

Mike - Are you saying that IF O2 sensors are suspect it would be the ones UPSTREAM of the cats?
 
  #11  
Old 11-08-03, 12:29 PM
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Mike,

The statement about sensor before the cat controlling fuel curve is not entirely true.

I had a case on a 4cyl Camry setting a PO 171 lean code. This car had all kinds of updates, ECU, A/F sensor etc. This is a MAP sensor equipped engine.

After talking to a product engineer he wanted to know what the voltage of the O2 after the cat was going down the hiway. I said whats that got to do with fuel curve...it just monitors the cat to see if it's working. Well the voltage was just about 0 volts. The voltage was steady but not high enough, should be steady around 0.5-0.7 volts.

After replacing the downstream O2 the voltage came up to 0.5 volts and the PO 171 never set again.

The reasoning behind this is some ECU's use the downstream sensor as a final check of A/F ratio besides it's primary cat efficiency checker. If it sees 0 to very little voltage it thinks the engine must be lean and overides what the primary O2 is reporting and sets a lean code.


Larry
 
  #12  
Old 11-08-03, 12:35 PM
Joe_F
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TG:

Wait for the scanner. In fact, you may have to drive around with it plugged into to simulate what is going on for the scanner. This will help pinpoint the problem.

I agree with some of the other suggestions. They cannot hurt. But, I think we need to talk to this vehicle's computer to hunt this down.
 
  #13  
Old 11-08-03, 09:59 PM
mike from nj
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larry, i think we had the same problem but with a different explanation.

once in a (dodge) fuel class, we were told to always watch all the sensors, as the downstream will also modify the upstream's target voltage, if it (downstream) acts up, it can send the upstream's target to .00 or 1.0 depending on which way it went, and from there, if the actual upstream voltage reading never crosses it's target in a set amount of times, the pcm thinks it stuck lean or rich, when in fact the problem is with the downstream.

i think that's basically the same result as what you said.

i've never seen or heard of it happening to anyone though (you're the first now)and on my limited gm experience, i've never even heard of that potential to happen, and on a tech 2, i can't remember seeing an upstream 'target' voltage. maybe it's there, but not shown, or maybe it has the same strategy, but we're not told about it.

it's good that someone pointed that out though.

davo??
 
  #14  
Old 11-08-03, 10:28 PM
mike from nj
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speaking of Tech 2s, if you have any access to one (mr. tow guy), it probably could pinpoint the problem down to an exact sensor. when the problem acts up, hit the 'quick snapshot' button, you then have a complete data recorder for all the sensors on the vehicle, and with an update rate of a couple times a second, you can then play it back and watch each frame and compare sensor readings in the same frame of time, much more accurate than a human eye can see while watching the scanner while driving. i've found problems that i would have never found other than guessing using this before.

unless the timing was off, and is already fixed.
 
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Old 11-09-03, 01:09 PM
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TG:

I'm interested to know the fix too.
 
  #16  
Old 11-09-03, 01:54 PM
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Mr TowGuy lol,clean the maf sensor also won't hurt something free to try,if you lived near pitts.Pa I'd lend you the vehicle data recorder so we could see some readings.I got a feeling it's not O2 sensor related,tps,maf,map,iat,coolant,speed sensor,knock sensor maybe.Any sensor on the voltage reference line can alter all sensors on the line.GM uses for the most part common sensor ground and reference voltage.Either one goes south they can all read bad to the PCM but the PCM doesn't always know this.Give us some info Mr Tow Guy you have us curious now.Does it still have a Delco battery?lol
 
  #17  
Old 11-10-03, 04:50 AM
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Was gone for the weekend, so don't know what the partner accomplished yet.

I confess I don't know what scanning equipment we will have access to.

It's got some big comercial batteries that reside in one of the toolboxes on the backend. Don't think they are Delco's.

Davo - the weather's better down here, 71 deg @ 7:45 am; need a vacation? LOL
 
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Old 11-10-03, 05:38 AM
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Ya, brrrr. Looks like I'll be firing up the snowblowers early this year. LOL. I went to get something out of the trunk of the car a little while ago, freezing! LOL.

Let us know what scanners you are using. I believe the problem will becoe clearer with some data.
 
  #19  
Old 11-10-03, 05:41 PM
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just wondering about fuel pressure was it checked when the vehicle was under a load and acting up if it wasnt checked this way it should be, hook up a guage where you can read it while driving the vehicle loaded.
sounds like a very basic fuel supply problem to me.
 
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Old 11-11-03, 04:07 AM
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I think the coomon scanner in this area is Snap-On.

The under-load fuel pressure test is on our list of things to try (soon!), bejay. Lately we've had about 5 minutes a day to work on it because of the work load.
 
  #21  
Old 11-11-03, 06:05 AM
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Let us know what you find.
 
  #22  
Old 11-11-03, 08:48 AM
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There'll be a brief (LOL!) pause while tow_guy and co. work on some of the suggestions. If you don't see me post the outcome, it's because we're still working on it. If/when we finally nail it down, you guys will be the second to know.
 
  #23  
Old 11-11-03, 09:08 AM
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This might help for starters (from the GM shop manual for this thing):

Lack of Power, Sluggishness, or Sponginess
Checks
Action

DEFINITION: The engine delivers less than expected power. There is little or no increase in speed when partially applying the accelerator pedal.

Preliminary Checks
Refer to Symptoms .
Compare the customer's vehicle with a similar unit. Verify the customer concern.
Remove the air filter and check for dirt or a restriction.
Check the transmission shift pattern and the downshift operation.

Fuel System Checks
Check for a restricted fuel filter, contaminated fuel, or improper fuel pressure. Refer to Fuel System Diagnosis .
Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .

Exhaust System Checks
Check the exhaust system for a possible restriction. Refer to Restricted Exhaust System Check .
Inspect the exhaust system for damaged or collapsed pipes. Inspect the muffler for heat distress or a possible internal failure.

Additional Checks
Check the Control Module grounds for being clean, tight, and in their proper location.
Check the EGR Valve for being open or partially open all the time. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
Check the Torque Converter Clutch (TCC) operation.
Check the A/C operation. Refer to Air Conditioning (A/C) Compressor Control Circuit Diagnosis .
Check the generator output voltage.
Check the Knock Sensor (KS) system.

Engine Mechanical Check
Check the engine for the proper operation of the following items:

Engine compression
Valve timing
Improper or worn camshaft.

----
I'm sure I can find plenty more in this thing I've got here, just tell me where you're going and what you find .
 

Last edited by twelvepole; 01-05-04 at 04:03 AM.
  #24  
Old 11-11-03, 02:22 PM
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Okay, I lied; I'm posting again and it isn't fixed yet:

Senior partner got in a little t/s'ing today, at least enough to apparently eliminate the fuel delivery as a suspect. Drove it with pressure gauge installed and didn't see any pressure drop associated with the power loss, plenty of pressure.

Also got a quick check on a scanner (in shop, not on road). No codes and, if I understood him correctly, only negative indication was something related to ignition module (that was the $100+ one we weren't in any hurry to try). New module had no effect.

Thanks for the laundry list, Joe.

The battle continues.
 
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Old 11-11-03, 06:06 PM
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if anybody else has any suggestions, this engine should be your old school iron block, iron heads, vortec-style injected 5.7
 
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Old 11-12-03, 06:09 PM
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Aaaaaaaaaaaand we MIGHT have a fix! But I think I'll give it another day of driving before we pronounce it fit. Gives you all a chance to post the "I'll bet it was the _______" items. ONE of you guys was really in the ballpark in a related sort of way. Tune in tomorrow, same bat-time, same bat-channel.
 
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Old 11-12-03, 08:28 PM
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"I'll bet it was the _______" items


one clogged cat?? (out of two)
 
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Old 11-12-03, 09:34 PM
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Ok, after reading this...I'm going to hazard a guess that the MAP sensor or vacuum line had problems, OR the TPS sensor was bad (although that probably would have made a trouble code).
 
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Old 11-13-03, 09:04 AM
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And the answer apparently is (drum rollllllllllllllll): Knock sensor overly retarding timing. As I understand it the computer will go to as much as 16 deg retarded to counter knocking. Imagine setting an old style distributor for that much retard; my guess is it would be PRETTY sluggish. Right now he's just running with the sensor disconnected with no negative effects and the power loss seems have vanished.

Congratulations, Davo, on being the closest.

On an entirely different note, I'm going to be doing something on my wife's '95 Camry that I've NEVER done in 25+ years of Toyota owning/driving: Replace a starter.
 
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Old 11-15-03, 04:00 AM
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Unhappy

Wellll, so the problem is 90% fixed. Most of the power loss is gone, but periodically it will show up. I had to drive that truck yesterday when the brake master failed in my truck and in about 6 hours of driving it bogged down twice, both times with vehicles on the "hook" on straight and level and cruising. First time I was doing about 45 and the second about 55. Both times the problems cleared after a few minutes. Partner says that when it does it he's been able to get it to clear by just taking his foot off the gas and letting it coast a ways.

Still scratching our heads.
 
  #31  
Old 11-15-03, 11:40 AM
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When the pump was replaced was the connector in the vehicle harness also replaced?Also how many miles on the truck?
 
  #32  
Old 11-16-03, 12:28 AM
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I agree with where Davo is headed...sure sounds like a fuel pressure problem. Still, it could be a bad spot in the sweep of the TPS. Moving the pedal could be re-connecting the contacts in it. I think I would be looking at fuel pressure some more though.
 
  #33  
Old 11-16-03, 04:27 PM
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Couldn't tell you right off the top of my head what the mileage is, but around 125k. Just getting broken in; my W-4 has 365k.

Fuel pressure not an issue. Maintains 60 psi throughout operating range and shows no drop when the power loss/bogging down comes in.

TPS also not a factor; been replaced, negative impact.

Does the bog down thing several times a day, always (as near as I can tell) between approx 45-55 mph. If I let my foot off the gas for a few seconds and coast, it's gone.
 
  #34  
Old 11-16-03, 09:04 PM
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Ok...one thing that happens when you coast for a second and then throttle up again is you create a large pulse in engine vacuum. I don't know if you have an automatic tranny or not, but I'm wondering if maybe you have a lockup converter that has a vacuum cut-out switch. If the cutout switch were bad or sticking, the drastic changes in vacuum could be getting it unstuck and allowing it to come out of lockup.

Also, another thing that comes to mind is the MAP sensor, which is also vacuum operated. Check the vacuum line going to it and make sure it isn't soft or cracking/splitting. If you have a known-good MAP sensor, it wouldn't hurt to stick it on there and see if it makes a change.
 
  #35  
Old 11-17-03, 02:18 PM
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Tow Guy have you guys cleaned the injectors on this?I'm wondering if they are just slightly restricted causing the truck to go lean.Did you guys pull the dist and check that gear?I f the gear is worn you can get a misfire you may not feel or set codes.They have been known to get bad enough to slip and cause no starts at less miles.
 
  #36  
Old 11-18-03, 09:55 AM
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Negative on the injector cleaning, haven't tried that as yet because of the intermittnet nature of the problem it seemed a little remote. Of course after we start to exhaust everything else we'll be trying a lot of remote stuff. Distributor checked, no problems noted.

I'm getting a little more "data" on the problem this week because I smoked a wheel bearing on my W-4 bad enough to require spindle replacement and I'm driving the problem child.

Really no additional insight into the characteristics of the problem beyond what has already been described.

Strange you should mention transmission, Cheese. I have it in mind to talk to our favorite tranny shop at my earliest convenience. Was going to ask basically if anything tranny problems would manifest themselves in the manner of our problem. While I was mulling that over I came across something that the truck is doing that may or may not be related: When your cruising along (and it's especially noticeable at low speeds, like 20-30 mph) and you take your foot off the gas, the truck almost maintains whatever speed you were at. It does gradually loose speed, but the lack of instant decceleration is noticeable. As I was poking along in a mobile home park at 20 mph earlier today I did a little more testing. As expected, when I took my foot off the gas, the truck continued to roll along at about 20 mph. BUT, here's the interesting part: While in this "foot off gas-still going 20 mph mode", I pushed the gear selector up into neutral and could easily hear the engine speed drop all the way to idle. Tried it a couple of times and it does it every time. AND if I then pull the gear shift back down into drive, I can hear the engine speed up to where it was previously. Weird.

Now, since this isn't my regular truck I have no idea if that is normal or not. The only thing I could think of that could cause that under normal conditions would be if the engine/drivetrain was designed to get a "bump-up" in rpm when put in drive (my W-4, for instance, does this when you turn on the AC; it does NOT bump up engine speed when shifting into drive).

Can't remember if MAP was looked at or not, willhave to quiz the partner.

And so it goes........
 
  #37  
Old 11-18-03, 07:15 PM
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The raise in RPM is probably normal...you have a connection between the engine and the wheels, even though it is a fluid connection, it still affects engine RPM. If it is increasing more than normal when you put it back in gear, it could be that the clockup is already engaged. If it is engaging in the 20-30 mph range, then the pressure cutout switch would most likely be the culprit. Does it seem to be locking up at such a low speed?

I mentioned the map sensor because it senses engine vacuum. It is basically a vacuum gague for the computer to watch. If it has a vacuum leak to it, it tells the engine that you are throttling up and/or under a load, and the computer increases the injector pulsewidth and adjusts timing. If it does this when you are not under a load, or under less or more of a load then the map sensor indicates, then the fuel and timing get out of whack.
 
  #38  
Old 11-19-03, 04:51 AM
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Hunted down the MAP sensor for a look. It is direct mounted to the manifold/plenum, so no vacuum line involved. Pulled it out and applied a slight vacuum and it held. Have to ask partner, but I think it is one of the items he swapped out already. Gaining power back on coast (high vacuum) is rather suspicious for sure.
 
  #39  
Old 11-19-03, 11:26 AM
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my two cents worth

it does sound more like fuel than anything else...and i know you guys checked fuel pressure and it's good....but fuel volume is equally important...don't know if ford gives specs for that make and model (some they do...and some they don't) but our rule of thumb in the absence of an OEM spec is 1 pt in 30 seconds...the fact that the problem goes away when you lift just reaks of fuel...sure wish I could drive it around for you...sure helps in a transient problem like this one...best of luck!
 
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Old 11-19-03, 11:41 AM
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carguyinva

I have found a volume test necessary when you had 5-7 psi pres.

Todays 45-50 psi systems you will see the pres drop if there isn't enough volume.

It may only drop 15-20 lbs indicating not enough volume.

Larry
 
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