Loss of Power

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  #41  
Old 11-19-03, 11:52 AM
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larry...

...thanks for that tip...i guess at this point they are probably double and triple checking some things and volume is important so i just thought i'd throw that out...as i said, test driving it would help me to understand the problem better and possibly give advice...hey towguy...here's an idea...get a propane bottle...a small one like for a little grill and get/make a regulator for it...run a piece of hose out into the induction system...road test it and when you get it to do its thing, have your riding partner give it some propane while you keep it doing its lack of power thing and see if giving it some fuel clears it up...if it does, then you've eliminated all those ignition possibilities...then you can concentrate on whether it's an input or output thing relating to the fuel system only...
 
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  #42  
Old 11-20-03, 05:21 AM
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Strangely enough, I went essentially the whole day yesterday without noticing it. Only difference from other days is we had some light rain. Only time I noticed it was first run of the day. Had to go about 10 miles down the Interstate. Initially had trouble getting it to go over about 63-64 miles an hour, but by the time I had to exit I was cruising along at just over 70.

Got my W-4 back on the road now (king-pin mounted front spindle replacement, WOO-HOO), so now partner has the problem child back. He mentioned in passing, when I was talking about the problem going away when you coasted, that he was also able to make it go away by slipping shifter up into neutral and revving it a couple of times.

Starting to think the computer is doing something wacky, but have no direct evidence pointing that way.
 
  #43  
Old 11-20-03, 10:04 PM
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Well, I'm not thinking lockup converter anymore if you are able to get it up above 60 before having much problem. At 60 you should be able to continue accelerating even if the converter was locked. Wierd problem.
 
  #44  
Old 11-21-03, 04:08 AM
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Almost at the point of having to wait for something to totally fail, which would be nice - much easier to track down the failed part than one that works 95% of the time.
 
  #45  
Old 11-21-03, 07:00 PM
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Okay, okay.....I just read the whole thread.
get a fuel pressure gage, a good one. It should be a high pressure gage, one that goes to 100 psi. (if this an 'R' motor, which is what I think it is...). Connect it to the engine, and leave it there. Make sure it isn't pinched or kinked anywhere. Keep a fire extinguisher handy. Drive the vehicle. When it is in a failed state, observe what the fuel pressure gage is doing. It should not fall. in fact, it should go up when u step on the gas. You should be able to hold this thing to the floor and the fuel pressure gage should not fall 1 psi. Let us know....the spec is 62 key on, engine off, probably around 55 at idle. Whatever it is at idle, it should go UP when u step on the throttle. If it goes down at all.....I mean 1 psi, you have a fuel delivery problem....
 
  #46  
Old 11-22-03, 12:43 PM
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Negative. Better read the whole thread again. Been there, done that. Holds at approx 60 psi. Pump and filter already replaced (pump had a bad connector and apparently had no bearing on the power loss problem). Don't even suggest replacing it again, it's like $300.
 
  #47  
Old 11-22-03, 11:15 PM
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what if you tap the brake pedal when the problem is occurring, just enough to light the brake lights. this should unlock the torque converter and possibly eliminate it? maybe watch for a slight rpm rise as it unlocks?

also try giving the cat converters a kick and listen for any rattles.

'bigguy', this is an average 'vortec' 5.7 (i think----i hope)all the ones i've seen were anyway.

ps. welcome back, bigguy
 
  #48  
Old 11-23-03, 07:02 AM
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Tow guy, was the 60 psi holding while in a failed state? This is key......knowing what the pressure is while not in the failed state is not going to be helpful,,,
 
  #49  
Old 11-23-03, 08:40 AM
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Yes, 60 psi across the board; doesn't flicker one iota.

Converters are not a problem.

Will have to try the brake tap routine.
 
  #50  
Old 12-02-03, 06:29 AM
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Tow Guy, I think I'm fighting the same thing in my '89 burb, 350, 700R4, R1500, TBI
If timing set correctly, poor power at highway speeds. If timing set advanced highway power is OK and get poor idle and smells really rich.
Left timing advanced, disconnected O2 sensor to put in Open Loop results in highway power ok and good idle. Been running in open loop since Sunday (11/30) and no problems noticable with mileage yet. I can't figure out what's happening in closed loop.


No Codes...Temp sensor agrees with thermostat and dash gauge, EST checks OK with volt meter, no crank position sensor on this model, Knock Sensor working (my cheap POS, OBD1 scan tool won't check knock), Ignition module checks OK, Distributor gear and shaft are fine, O2 MV moving correctly (also shows propoer lean/rich cross counts with poor idle, so replaced) reading OK, Map MV OK with engine running and off (key on), TPS OK through-out range, Found bad EGR soleniod - replaced no difference, ran my ECM & PROM on another burb and it ran like a clock. Have not checked cylinder compression but did check vacuum at idle and above for clogged converter and / or valve timing and found no problem.

Please let me know if you find anything. I can get away with running an old truck here without the ECM controlling properly. They don't check any autos prior to OBD2, but I'd rather have it right.
 
  #51  
Old 12-02-03, 07:56 AM
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Only thing we've found lately is that we had a bad battery. The truck has 3; one in the usual location and two auxilliary ones in one of the tool boxes on the back-end to provide additional reserve for the hydraulic system (electric over hydraulic). One of the box-mounted ones was half dead so we replaced it last weekend. Doubt if that will have any effect, but at this point I'm willing to believe ANYTHING will fix it.
 
  #52  
Old 12-03-03, 10:12 AM
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Well did the battery help?

Did you look at EGR??
 
  #53  
Old 12-04-03, 04:00 PM
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Don't think the battery had any effect. Haven't jawed with the partner in a couple of days to see how it's doing.

Have not done anything with the egr. Refresh my memory on the theory of how that would affect performance in this particular matter? Still getting no codes to indicate any definite problems.


tg
 
  #54  
Old 12-04-03, 05:54 PM
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Too little EGR flow allows combustion temp to get too high under engine load. Rise may or may not show up in coolant temp. High cylinder temp can cause spark knock, Knock sensor retards your timing trying to eliminate the detonation it hears.

Also, I've heard of bad ECM grounds causing problems on a pre-OBD2 GMC truck. I guess it could affect an OBD2 as well.
 
  #55  
Old 12-05-03, 06:37 PM
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Mr Tow Guy do you know anyone with a tech 2?I still think you need to check CMP retard.Got a gut feeling on this nothing concrete.Need to drive the beast under the conditions and note the cmp retard.That controls all the fuel curve.If it's more than plus or minus 2 it's not good.
 
  #56  
Old 12-07-03, 06:15 AM
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Will have to nose around to see who's got what for scanners.
 
  #57  
Old 12-13-03, 02:50 PM
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If this is the same as a 5.7 vortec than it is the fuel pump. Did you replace it or just fix the wires. they go bad alllllllllll the time on chevy trucks and 99 was a real bad year. You cant rely on the fuel pressure test with cpi injection. Dont ask my why but it has something to do with the poppet valves and way they open and air bubbles in the fuel lines . Also on CPI there is a wiring harness on top of the intake that connects to the injectors. I have taken these apart and found injector prongs bent. only 7 out of 8 injectors firing and no misfire codes and no miss felt at all, just slight loss of power. Run engine with fuel pressure gauge in bypass and a clear tube coming out of the gauge and look for air bubbles. if any air bubbles replace pump module.
 
  #58  
Old 12-14-03, 07:55 AM
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Pump was replaced to the tune of about $300 (wholesale). We're about as convinced as we can be, given the pita this has been, that it is ignition-related vice fuel. He had a real bad time with it the other day and on a whim decided to try moving the distributer. Didn't cure the problem, but there WAS some improvement, which would tend to point toward a timing problem.
There's still a MOUNTAIN of things we haven't tried, mainly because there are only so many hours in a day and days in the week.
 
  #59  
Old 12-14-03, 09:43 AM
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The distributor doesn't control ignition timing on this engine the ckp sensor and pcm do that.Rotating the distributor changes the camshaft position retard which controls when the injectors fire.That is why I said get the camshaft position retard to + or - 2 degrees.It is very important for the fuel to hit the cylinder at the proper time.Will this fix the truck I'm not sure but I can say it needs checked.Mr Tow guy you have to know a local GM tech whom can check it for you don't you?No I'm not a preacher man at least not yet.lol
 
  #60  
Old 12-15-03, 03:57 PM
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Right, understand about the computer controlling the timing. Not sure what his motive was in going there beyond, "well, what HAVEN'T we tried yet".

Got lots of people scratching their heads, GM and non-GM alike.

May be getting worse, which would be a GOOD thing; if something would just go ahead and FAIL it would be much easier to track down. At least one time today he said he had a hard time getting it to go over 35 mph empty, but was okay most of the day. There never seems to be any particular common thread running through when it may or may not decide it wants to bog down. I haven't driven the problem child myself in a couple of weeks (thankfully).

Haven't come across anyone yet with a Tech 2. Did I mention it's warm and sunny down here? LOL
 
  #61  
Old 12-15-03, 04:26 PM
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Are you guys sure the air intake isn't restricted?Try removing the air inlet duct from between the air cleaner and MAF sensor and give it a spin.
 
  #62  
Old 12-18-03, 08:37 AM
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No intake problems.

Bit the bullet yesterday and I ran calls all afternoon while my partner imposed on one of our "A"-list garage owners. Apparently they did some extensive troubleshooting and have narrowed it down to MAP sensor circuit while eliminating some of the other question marks like cam timing, etc, mentioned in previous posts. Don't have all the details on what they found, but apparently they tried different sensors with no change so we'll be looking @ wiring, computer, etc from the standpoint of the MAP. Still haven't figured out why we get no check engine light on the beast, but there may be light at the end of the tunnel at least.

Told Santa Claus all I wanted for Christmas was the fix for the truck.
 
  #63  
Old 12-18-03, 10:53 AM
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another thought...

...with respect to the fact that changing the timing seemed to help. does that vehicle have a knock sensor? I've seen knock sensors and misrouted sensor wiring cause major spark retard. all this would happen without setting any codes since it would essentially be doing it's job. just a thought...
 
  #64  
Old 12-18-03, 03:51 PM
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At one time the knock sensor was a suspect for reasons as you suggested but we eliminated it as a likely suspect by both disconnecting it and also physically removing it and tieing it off away from the block.

Okay, on to today's additions to the known and unknown:

Here's what my partner related as pertains to the lengthy troubleshooting yesterday. They ran the entire list of systems and everything came back in-range/operating properly EXCEPT MAP. Apparently (and I may not be relating this 100% accurately)when they applied a test vacuum (20 in, I think), the computer readout was much lower, something on the order of 9 in. As I understand it they tried several sensors (none, admittedly, was new, but what are the chances that 3 or 4 would all read bad the same? PLUS, never a check engine light/code set!) and got similar results. SO, today we did some wire-shooting to see what we might find. So far negative results: Input voltage to sensor approx 5.1-5.2v key on/eng off, good continuity on the ground, and we also checked the signal line from the sensor connector to the computer connector, good continuity-negligible resistance. Output voltage from the sensor also in the 5.1-5.2v range whcih I'm assuming is normal and that as vacuum increases the voltage decreases, right? Which pretty much leaves us at buying a MAP sensor on a hunch (>$60+) or considering a computer malfunction.

Question: If we set up to read signal voltage during operation, is there a set range that we should expect to see? Obviously we'll be getting something between 0 and 5v, but is there some direct relationship, i.e. 10 in vac = 4.5v, 15 in vac = 3.8v, etc?

I think we may r & r the sensor just for peace of mind and to eliminate it once and for all as a suspect, but I'm also thinking that we can read voltage as we drive and see if there is some weird fluctuation when it starts to bog down. Taking a reading from the computer wouldn't seen to tell us anything since we already know the computer at least THINKS the MAP signal is low.

Comments?
 
  #65  
Old 12-23-03, 12:29 PM
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Hang onto your hats, folks, because the beast is FIXED!

Discovered the problem pretty much by accident (as I suspected all along would happen sooner or later).

And the answer is.....a short in the tow lights/tow light cable.

So now that everybody is saying "HUH?????", here's what happened.

My partner was having trouble Saturday with his turn signal fuse continually blowing. During a little roadside troubleshooting he noticed the tow lights weren't operating properly - no brake lights or turn signals. Sooooo, he made the connection that he probably had a short in them somewhere (they get pretty beat up in normal use) and simply unplugged them until he could get time to track down the short or whatever. Low and behold, he pulls away from the side of the road and before he knows it he's doing 65 mph. Big surprise; he hadn't been over 50 in about a month. Been two working days now and not a single sputter, bog-down, nothing! Weird! Must have been causing some kind of stray voltage to get to the computer or SOMETHING.

Many thanks for all the assistance from everyone and, of course, I realize that I can never post another question concerning one of the wreckers because the first thing anyone will say is "did you try disconnecting the tow lights.........?". LMAO
 
  #66  
Old 12-23-03, 01:48 PM
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I'm glad that thing is finally fixed.Sometimes it's weird.I just fixed a car with the complaint that remotes don't work,radio inop,reverse lights always on,no daytime running lights and all guages inop including the speedo.A new radio fixed the car.The radio was preventing all the modules from communicating.Tow Guy I hope that beast stays fixed.lol
 
  #67  
Old 12-23-03, 05:32 PM
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Thats one for the basics

The lights shorting and lowering the battery voltage would have showed up reading the B+ at the time of failure.
 
  #68  
Old 12-23-03, 10:19 PM
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that 'add on' wiring will bite you every time!!

i really wonder what/how it was tied in. maybe it was feeding a little voltage to it's ground, and was sharing the ground from the fuel pump, could it have been slowing the pump down by putting a few volts up the ground wire? i've seen feedback from brake lights affect fuel pumps before

as long as it's fixed, that's good.
 
  #69  
Old 12-24-03, 08:19 AM
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Beats me. The tow light receptacle comes as part of the "back-end" and was wired into the chasis electrical by the manufacturer (Century, now Miller Industries) at time of installation ('99). As far as we know it's hooked in the same as you would with any aux lights, i.e. for a trailer. The short is obviously in the tow light cable vice the truck wiring, but darned if I know how that would get back to screw up the computer. During troubleshooting we had checked bat voltage (finding one of the three batteries going bad, appparently with no relation) and alternator charging, but of course we never did it while in "fail" mode. Have been saying for weeks that it would probably turn out to be something really stupid and we were right.

Think we set a record on this thread with # of "views"!
 
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