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1997 AWD GMC Safari
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Discussion Starter · #1 ·
I have an intermittent misfire that is usually unnoticeable, but yesterday had me thinking the van was going to stall.

So, time to deal with it!
1997 AWD 141k miles

It has not thrown a code for this, but I see misfires logging on my bluetooth scantool, and it has had p0300 (random misfire), and yesterday p0172 and p0175 (fuel trim rich for both banks) as pending codes - although they never triggered the SES light.

The misfires do seem to be at every cylinder, and happen under different conditions. I'd say it most consistently happens when I take my foot off the gas after accelerating.

Within the last 20k miles, pretty much everything is new.
Coil, plugs, wires, cap, rotor, distributor, map, pcv, vac lines, fuel pump, injectors (upgraded to mpfi), plenum gasket, intake manifold gasket, etc.

Rented a fuel pressure tester kit from the Zone. I checked the numbers, then cleaned the MAF, cleaned the fuel pump ground, and replaced the fuel filter. Got the same readings before and after -and still misfires.

When I prime the fuel pump with key, it hops to 58psi, then drops to 55 when the pump stops.
Over the course of 15 mins it will lose ~10psi slowly.
When the engine is running at idle, it reads 48psi, and goes up a bit with throttle.
When logging misfires on my scanner, there is no jump of the needle or anything obvious on the pressure gauge.

The fuel pump was a new NAPA pump I had installed in 2017.

My factory manual says 60-66psi is spec when priming the pump with the key. I believe the mpfi has a slightly lower value than the system that came with the ''97, and when I did this test after I redid the injectors/intake manifold gasket in early 2018 I'm pretty sure I had the same readings.

Should I be considering the fuel pump as a potential source of this issue? Any suggestions on where to look next?
:ty:
 

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I had this exact same issue on a Van the other day same psi. I was lucky enough to have another Van come in that had a new pump in it and the fuel psi was 60psi running vs 48psi running and would increase a bit with throttle. It also had a P0300.
 

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1997 AWD GMC Safari
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Discussion Starter · #3 ·
Thanks for the input Copper - so you'd put your money on fuel pump? I luckily have a spare I got on a closeout last year, so I could make it happen.
 

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1997 AWD GMC Safari
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Discussion Starter · #7 ·
I had a compression check done like ~15,000 miles or so ago. It was pretty much perfect, and seemingly nothing out of the ordinary since then to wreck all the cylinders, so I am really thinking I am all set there.
 

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05 Astro AWD
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Those numbers just seem a little low. I'm leaning towards the fuel pump.
 

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1997 AWD GMC Safari
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Discussion Starter · #9 ·
Well, update time.

Replaced the fuel pump - it was some brand I've never heard of.
Replaced with Denso pump.
Fuel pressure numbers afterwards are exactly the same as before, and the misfire persists.

The random misfires will happen at any time, and aren't very noticeable. But I have found that I pretty much always get them when I press the gas pedal and then release it quickly - in the gap when the rpms are descending from 2000rpm or whatever to idle level, misfires will happen.
It also does not like stop and go traffic. Idling or acceleration from a stop will frequently cause noticeable stumbles or bogging. So, I'd say it's worse when hot.

I did a lot of searching and found a lot of possibilities for these symptoms. So,today I swapped in my spare camshaft position sensor, ignition control module, and crankshaft position sensor one at a time, with no change.

That leaves these as possibilities - -
- Ignition coil dying - going to buy one tonight and replace - worst case I'll have a spare
- Coolant temp sensor - have a spare I can try
- O2 sensor - bank 1 sensor 1 is only original left - though voltage readings on the scanner are doing the normal high to low oscillation
- Catalytic converter - can pull the O2 sensor to see if a clog there is causing an issue
- EGR - put in new screened gasket and cleaned it 20k miles ago, so not super high on this
- Throttle position sensor - can test this of I get there, I don't think the symptoms are very consistent with an issue here since it will misfire at idle
- Fuel leak in line after regulator and before it comes out of plenum
- Timing/CMP retard off maybe? Timing is +20* on the scanner
- Air temp sensor maybe?
- Idle air control valve?
- idk

So, that's where this is at, for all of you closely following along out there. Hope the coil is the issue lol.
 

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2000 Lifted 4x4 Astro 92 V8-350 Shorty
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That's a nice list! :D
Cross off fuel pump..? (I think ... pressure still seems low)
* You left out "fuel injection spider" as a possibility
* Didn't see ECM on this list either (not sure if it could cause misfires)
Both of these big items I've replaced recently.

I'd certainly tackle the easier less expensive stuff first.

Have I mentioned lately how much I love my carburetor and HEI on my 92... :D
(I have not one single item on your list, except coil built in HEI)

SportsBoy said:
- Air temp sensor maybe?
OK, I can field this one... NO.
Sidenote: I actually intentionally run mine disconnected.
Fools engine into thinking it's cold, boosting timing, and power.
It's an mpg improvement for me (under my circumstances)
(no misfires)
 

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1997 AWD GMC Safari
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Discussion Starter · #11 ·
Idk about the air temp sensor - just read that fixed it for someone in a different thread :shrug:

Spider is upgraded mpfi(?) one, new 20k miles ago. I am not really suspecting that because it doesn't seem like it would be causing random misfires in all cylinders, but am I wrong on that?

I know very little about the ECM (engine control module?) so I really hadn't thought of it at all.

I was thinking of how I was jealous of carbed engines today lol

I forgot to mention - inspected cap and rotor - they are new in the last year and have very few miles - they look new still so I didn't swap those.

New coil in, and no change - still rando misfires at idle, and when playing with the gas pedal in park. :(
 

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Your fuel pressure numbers are low. Did you replace the fuel filter when you replaced the pump? The pump's warranty will not be good if you didn't.
 

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1997 AWD GMC Safari
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Discussion Starter · #13 ·
I don't think I have any sort of warranty on the pump…

I replaced the fuel filter like 300 miles before doing this pump, so I didn't do it again.

When I did the fuel pump, I looked inside the tank and it was nice and clean - so I'm also not worried about the filter being clogged.
 

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1997 AWD GMC Safari
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Discussion Starter · #14 ·
Unplugged the throttle position sensor, mass air flow sensor, inlet air temp sensor one by one with no change, still random misfire.

I unplugged the MAP sensor though, and can't get it to misfire with it unplugged. It doesn't seem to run any better - if anything it seems rougher - but no misfires have logged with it unplugged.

Posted about my MAP-related readings here -
https://www.astrosafari.com/viewtopic.php?f=43&t=147382

Odd, huh?
 

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1997 AWD GMC Safari
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Discussion Starter · #15 ·
New MAP sensor in - no change!

Not sure why misfires would go away with it unplugged, when it isn't bad :shrug:

Swapped over a known good cap and rotor and no change either. Swapping back to the ones I had on originally I didn't note which direction the rotor was facing :banghead:
Spent a while googling and then figuring out how to get the engine to TDC so I could use the alignment mark on the distributor. Wasn't sure I was doing it right until I started it up and it idled fine - minus tye random misfires of course.

So now I am most suspicious of a leak inside the plenum that is affecting all the cylinders - but who knows really. I also have these on my list as remaining potential causes -
- [ ] Coolant Temp sensor
- [ ] Egr
- [ ] O2 sensor
- [ ] Cat converter
- [ ] Timing/CMP retard?
- [ ] ECM?
- [ ] Idle air control valve?

I know nothing about those last three as of right now, but read them as potential causes in my research.

Getting close to bringing this to a shop :banghead:
 

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Discussion Starter · #16 ·
Quick correction - I swapped the knock sensor, not the crankshaft position sensor. Got them mixed up.

Just removed the crankshaft position sensor and looked to see it it was contacting on the inside, as some people have had issue with. It was tight and unmarred, so I put it back on. Also wiggled the wiring and tapped it with the engine running - no difference.
 

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how about the ignition control module?? those can cause all kinds of problems when they act up...it controls the ignition coil....my 92 died on the highway with the exact symptoms of fuel pump failure....she choked and just quit on me..they overheat overtime and break down the small circuit board thats in there...
 
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