New Stumper

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  #1  
Old 02-08-18, 12:50 PM
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New Stumper

Okay, so after tearing my hair out for couple of months with my random misfire that's finally fixed and trucks running better than it has in YEARS. This is the wrecker 1994 5.7L Chevy TBI.

The stumper, though, is one that cropped up last year and comes and goes at random. What happens is I crank it up and it fires off goes to idle rpm(more or less) and then immediately dies. Sometimes a second try will duplicate and sometimes it will remain at idle. Applying a little gas pedal usually makes it go away. Have already eliminated the TPS and IAC. When I disconn the TPS, it starts and runs fine even disconn'ed. With the IAC it starts and runs at a fast idle. Most of the other components that might be suspect have very low miles on them, plus don't want to just throw parts since it's not costing me any money like the misfire was. Scanner tells me nothing, since it never does it when I'm hooked up and there's no codes.

Anybody ever run across anything similar?
 
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  #2  
Old 02-09-18, 03:49 PM
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does it occur when cold or warmed up or both?
would probably check the base idle speed with iac closed the engine should still have enough airflow around throttle plates to idle at a slow speed around 600 rpm if for some reason you do not have minimum idle speed the engine would be more likely to start and die as its completely relying on the iac bypass air to keep it running.
 
  #3  
Old 02-09-18, 05:30 PM
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Do you ever get a anti-theft or security light blinking on the dash when it shuts off? I had a 1997 GM product that had a fault with the Passlock anti-theft system, and it would start, then immediately stall.
 
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Old 02-10-18, 04:28 AM
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I had a Ranger about like that. I changed the temp sensor for the computer and it took care of it. It would always die the first time, I never checked for codes on that one. I just replaced the sensor.

There was an odd one, though on a Camry. It was a head ache that turned out to be nothing more than low coolant level. We figured the temp sensor wasn't submerged in liquid.
 
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Old 02-10-18, 07:39 AM
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Does it both warm and cold, perhaps more often when cold although the incidents are so far apart can hardly remember from one to another. Base idle checked during previous troubleshooting for the misfire. IAC has been swapped out during other unrelated troubleshooting.

It's a '94. The only anti-theft device it has is the door locks.

Temp sensor replaced, also during recent troubleshooting. I was really grasping at straws at the time. Also, I've had a CTS go bad before and it resulted in a total crank/no fire (and code set). It sets no codes except for an oddball trans code that's come back time after time for about the last 300,000 miles.

The entire ignition system is essentially brand new from troubleshooting the random misfire. Almost makes me think it's having a random event of dropping the injector pulse. Would be nearly impossible to diagnose if that's it; 99 times out of 100 I'm not at a location where I could troubleshoot it and it's not always immediately repeatable anyway. If I ever get the chance I'll check that, all I need is someone to gaze down the TBI throat while I crank to see if we're getting fuel delivery (fuel pump has checked good and filter is fresh).

Likely to be one of those things I just live with. Did it one time this week, Tuesday or Wednesday
 
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Old 02-10-18, 09:56 AM
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Everything runs on voltage/resistance. It could come down to a bad wire or poor connection. Sometimes ya feel like looking for a 1960's something.
 
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Old 02-10-18, 01:03 PM
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I think I would probably just live with it also, at least until it got a lot more consistent since it always starts back up, if the engine has 300k miles on it could be something even mechanical the computer cant compensate for like timing chain wear if it starts to occur more often maybe you will have the chance to do some more testing.
 
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Old 02-10-18, 06:04 PM
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I had a slightly similar situation a few years back with a 94 Buick Park Avenue. Iíd be driving down the road and it would die. Pull off the road, turn the ignition off and back on and it would crank right up. No trouble codes would set. It got so bad I could shift to neutral, turn the ignition off and back on and crank it back up without ever slowing down. After about 6 months it finally set a code (donít remember what it was now). When I followed the steps to troubleshoot the component, it was good. Turned out the ECM was bad. I replaced it and it never died on me again.

I'd think there's a good chance yours is bad but if it were me I'd obviously wait until I could prove it before replacing it.

Jim
 
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Old 02-11-18, 08:40 AM
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My tow buddy, who's a way better mechanic than I'll ever be, keeps reminding me I'm into uncharted territory with 300k miles on it. I'll post more if anything changes. Not even sure I could FIND an ECM if I needed one.
 
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Old 02-24-18, 06:47 AM
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It will be nice to see how this one turns out.
Sid
 
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Old 02-26-18, 05:07 AM
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So far my thinking is running towards fuel pump/solenoid/check valve direction. Nothing definitive so far, but R&R of pump is on my things-to-do list as soon as I get a free Saturday. The current one is just about at the historic average mileage at failure point.
 
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Old 02-26-18, 06:30 AM
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I once had a 75 Pontiac Gran Prix with a 400 CI Engine that was behaving like that . . . . it would even die on me while idling awaiting my turn at a Toll Booth.

After being annoyed with it for over a year, a Mechanic Friend diagnosed it as a whimpy braided Ground Cable from the Battery to the Engine.

The Ground Cable was actually a GM cheapened Aluminum clad with Copper held onto a Cast Iron Head with a Steel Bolt and galvanic corrosion from the aluminum had created an intermittent unreliable connection between the Battery and the Ignition System depending upon heat and humidity. It was probably just as intermittent at highway speeds but wasn't so noticeable as when it was just trying to idle.

Changed to a strictly Copper braided Cable and the problem was solved.
 
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Old 02-26-18, 09:07 AM
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You may want to monitor the fuel pressure when it acts up if you;re thinking that direction, TG
 
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Old 02-28-18, 05:09 AM
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Re: ground cables. Was in troubleshooting mix on some electrical issues not too long ago, so no issues there; I even have a secondary ground to the block.

Re: monitoring. Yes that would be nice, but not very practical. I don't have a schader valve in the loop so when I want to check pressure I have to install an adapter. With the proximity to the exhaust, not something I would really like to have installed for long-term. Also, this is a cab-over (Chevy W4/Isuzu NPR) so about the only place to mount the gauge would be on the headache rack out the rear window. And then, of course, I would have to actually REMEMBER to look out the back window every time I got in and turned the key.

Additional info: Smoked my tranny last week and had to be replaced. Good news was my favorite tranny guy actually had one on the shelf gathering dust due to a mix up with a customer sometime last year. Basically got it for cost and it's a bit of an upgrade, too.
 
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Old 02-28-18, 05:14 AM
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One of my lines of thought is that it is a check valve issue and occasionally after the pump does its ~ 2 sec run to prime, that it's not holding the pressure long enough for the engine to get up to speed and get the pump energized via its normal sequence (signal from ignition module to PCM that engine is turning), although you would think the short run period before it dies would be sufficient to get things up to pressure and running. Hasn't done it for a few days now, but I'm sure it'll be back.
 
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Old 02-28-18, 07:28 AM
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Re: monitoring. Yes that would be nice, but not very practical. I don't have a schader valve in the loop
I understand. Your logic makes sense. My mother-in-law had something similar on a 3200 in a Cutlass. I believe the fuel pump was what they replaced.
 
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Old 02-28-18, 08:54 AM
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Well, even if it doesn't fix the problem, at least I'll know I've got a good pump. Over the years they've lasted an average of about 50k miles and this one is up over 40k already. Will probably be the last pump I'll have to change.
 
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