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Old 11-04-2013, 03:11 PM
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2000 GMC sluggish on hot restart

2000 GMC 3500 work van with the 5.7 motor, 215k miles. On hot restarts it is very sluggish and sounds like it is misfiring for maybe 20 - 30 seconds (kind of hard to hear/tell from the drivers seat) then all is fine. No codes are present or pending.

Just replaced the 'spider' injection setup with the EFI style upgrade (PITA job), replaced one ratty looking vacuum line in the process but otherwise everything looked good. It still acts weird on hot restarts however.

Looking for any pointers to help diagnose this problem, going to put a fuel pressure gauge on it in the morning and try to borrow a Solus to get some live data and look at O2 sensor counts/coolant temps (hoping one of the sensors is bad).

Any help appreciated...

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Old 11-04-2013, 04:15 PM
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i have had some hot restarting on a few over the years. and one was the pos battery cable. new one installed and it worked better then before.
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Old 11-05-2013, 10:50 AM
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thx killer, will keep that in mind

a little more info;

fuel pressure is 55 psi with engine off, key on.
52 psi idle, 62 psi WOT

O2 sensors are switching, coolant temp sensor is working correctly

Has a steady 22" vacuum at idle

bank 2 fuel trims at -4, bank 1 at -1

I did notice that the EGR wasn't opening but not sure if it is supposed to just feathering the throttle in park....
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Old 11-05-2013, 11:44 AM
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Fuel pressure is OK, I think that you will find that your WOT and Key On ENgine Off pressure(with the fuel pump running) should be the same. Stock
should be 62-66psi.
If you stand there with the KOEO does the fuel pressure bleed off any furthur?

When was the last time teh plugs, wires and cap and rotor were changed? These Vortecs fire with a very hot coil and are hard on teh caps and rotors. I change mine annually and notice a difference in power at every change.
Fuel pump.

It didnt do this with the old CSFI, and only with the new MPFI spider?



Did you lube up the CSFI unit's seal before you slid the upper plenum unit down? That can be a *****, last time I did it I had some leaks found when testing with brake cleaner.

Anu codes?

peace
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Old 11-05-2013, 12:36 PM
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Quote:
Originally Posted by Hogg View Post
If you stand there with the KOEO does the fuel pressure bleed off any furthur?
no

Quote:
When was the last time teh plugs, wires and cap and rotor were changed?
new last fall

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It didnt do this with the old CSFI, and only with the new MPFI spider?
no, it did this with the old unit as well

Quote:
Did you lube up the CSFI unit's seal before you slid the upper plenum unit down?
yes, also tested vacuum at 22" - rock steady.

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Any codes?
no codes present or pending. Accd to the scanner, it has been over 30k miles since any codes were cleared.
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Old 11-05-2013, 12:49 PM
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I guess it's worth mentioning that the truck has been doing this for well over a year. Changing out the plugs/wires/cap/rotor did not help with the problem.
Best I can tell it is going pig rich for a moment after a hot start, just not sure why.

(tried to edit this into my previous post but no dice)
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Old 11-06-2013, 12:15 PM
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ok,,, it sounds like a crank sensor, if you have acces to a scanner,
on cold start up monitor the crank sensor and graph it, it should be a nice square wave signal and it will speed up or decrease according to rpms, after warm up , tried to start and monitor crank snsr, and monitor the graph see whats different and also the voltage on snsr, usually after warm up crank snsr gets hot and will start to miss the signal and it will misfire in random cylinders. and that will explain the no codes it didnt fail yet its on its way out ... hope this helps,....
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Old 11-08-2013, 04:42 PM
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In my experience crank sensors are a go or no-go thing. I had one go in one of teh 97 L31's and upon replacing it, still teh no start, until I removed teh new sensor and then pushed the sensor in all the way THEN tightened the bolt. Dont use teh bolt to draw the sensor in.

Pull the CKP sensor and see if its worn, I have seen them hit the reluctor.

81Camaro has a solid methodology above. COuld very well be whats happening here.

peace
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Old 11-08-2013, 09:11 PM
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It might be a while before I take time to look at it again. It is a work truck (small company - 2 man operation) and we need it to make money not be down for a small driveability issue when we're about to get real busy.
Definitely appreciate the replies and will post back if I find anything.
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Old 11-08-2013, 09:47 PM
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well you can not make money down the road when it is broke!! Cash spent know is better spent. then 3 months later and no cash to fix it!! Spend Money to Make Money!!
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Old 11-09-2013, 09:10 AM
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When it is hot and starts hard once it smooths out. If you shut it down and restart it right away does it do the same thing or is it ok. Does it have to sit for a while before it exhibits this behavior.
With that many miles I am wondering if you have lifters bleeding down when the oil is hot.

Also have you looked at the MAF reading on the scanner at idle to see what it is. Should be about 6 grams at idle.
When it it running crappy what are the short term fuel trims.
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Old 11-11-2013, 05:15 AM
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Truck is not hard to start, fires right up every time. What I meant by "sluggish" is that it won't accelerate worth squat for the first 20-30 secs after a hot restart. Acts like it is flooded. Havent tried to start it immediately after shutting it off to see if symtoms are present. Will try that later today.

MAF looked fine on the scanner, STFT were at zero (on hot restart), LTFT listed in post above.

My mechanic friends (that let me use their Solus at times) are stumped by this thing too....
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Old 11-11-2013, 12:20 PM
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well sounds like theres a couple things you need to check, on the solis,
lay out the coolant temp snsr, maf snsr, 02 snsrs frnt and rear, rpms, map snsr reading, and monitor the reading after warm up and taking off.. the long term trim on b2? i believe (-4) is taking away gas,,, so on that bank you have more than normal on the rich side, and thats long term , over a period of drive cycles, ill look into fuel injectors, i understand you just changed it, it cuold have a poppet valve sticking notorius for this, or maybe the primary 02 snsr is not registering correct, the stft should read something after warm up and not ( 0 ) ,,, this is the one you need to monitor since its whats happeing at that moment in the engine,
if its reads along the lines of any thing over -5 ,-6-7 and so on ,, means too much gas ,,, that would cause the sluggish feel , also check the plugs ,,, see whats going on ,,,sorry i re -read the post,,, forget the poppet valve, you said it did this before even with the old one, i would lean more toward the 02 snsrs... especially the primary ones,

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Old 11-11-2013, 03:25 PM
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Quote:
Originally Posted by Rhansen View Post
Truck is not hard to start, fires right up every time. What I meant by "sluggish" is that it won't accelerate worth squat for the first 20-30 secs after a hot restart. Acts like it is flooded. Havent tried to start it immediately after shutting it off to see if symtoms are present. Will try that later today.

MAF looked fine on the scanner, STFT were at zero (on hot restart), LTFT listed in post above.

My mechanic friends (that let me use their Solus at times) are stumped by this thing too....
Does it make any noise like low oil pressure. I am really wondering if the lifters are bleeding off. Also when you checked the short term fuel trims and MAF did you do it when it was experiencing the issue.
Also did you look at the fuel pressure when it was doing it.
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Old 11-11-2013, 05:13 PM
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With that many miles,could be a bunch of things.1 thing that comes to mind is for some reason the PCM maybe pulling timing due to actual detonation,or,false knocks from some other noise.That would explain why it only seems to happen after warm up.
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