Misfire is turning my hair grey!!!! Damn you truck

[DieselDetroit]

97 Tahoe 2dr 5700/4l60e
315,000 kms


Symptoms:
- misfire consistently under LOW load (lightly pressing accelerator) - makes me suspect fueling, injector, fuel filter, pump, etc
- goes away under high load or when accelerator pedal is completely left alone (coasting downhill or cruising uphill) - makes me suspect same as above
- flat stretches are the worst for misfiring
- I'm losing torque converter lockup because of it.... read somewhere historic misfires prevent tcc lockup
- here's the weird thing, check engine light is off!!! Today it blinked for the first time In 5000 miles..... and it is very rare. No active codes. No stored codes???? Wtf!! It's an OBD2, shouldn't it be stored even after the ignition is turned off
- I thought I saw coolant condensed in the tailpipe (orange crystals), I 'think' I smell coolant but my mechanic friend said it's just fuel.... suspecting lower intake gaskets?

My question is how to verify this without codes? I want to see what these misfire codes are....

 

[Spode]

Living the same thing right now. Around 1500 rpm. Shakes you apart under light acceleration. Several have said it is fuel injectors. One gets stuck open, floods out cylinder at low rpm is fine when under serious load because that's what it would be calling for. I've got a 97 also. Guess the factory setup was so bad, gm came up with new style (MFI) that is an improvement, but actually cheaper than original style. Thinks that is going to be my weekend project. Hope it's the right fix.

 

[DieselDetroit]

Let me know how it goes. Injector? Or injectors? I thought there was one.

Also, I looked at the intake gaskets, it looks like if there was a coolant leak that it would go into the crankcase, not the intake system...... me confused

 

[skylark]

Vortecs are notorious for bad distributor caps.

 

[DieselDetroit]

Vortecs are notorious for bad distributor caps.

Opened the distributor up last year and the cap looked ok

Also, would it make sense with the issues I'm having? Being load related? Going away under throttle?

Also, why doesn't the computer store misfire codes???

Does anyone here have an instance where a bad intake manifold gasket lead to misfiring? I looked at the gaskets and the manifold and find this nearly impossible. Also, surely I would see an external coolant leak or coolant in the oil if this was the case.

 

[c1500deathtrap]

More information is needed. Do a compression test to rule out mechanical issues like worn rings or burnt valves etc. Then figure out if its running lean or rich when your misfire occurs maybe get a fuel pressure reading when the misfire occurs. Essentially narrow it down to fuel or spark while hopefully eliminating the possibility of mechanical issues.

 

[DieselDetroit]

More information is needed. Do a compression test to rule out mechanical issues like worn rings or burnt valves etc. Then figure out if its running lean or rich when your misfire occurs maybe get a fuel pressure reading when the misfire occurs. Essentially narrow it down to fuel or spark while hopefully eliminating the possibility of mechanical issues.

Compression test results was 7 were 190 +\-5 and one was around 205

How would I get a fuel pressure reading when it misfires?

Also, wouldn't the fact that the misfire is load depending indicate it's air/fuel related?

Also, my neighbor insists it's the intake gasket leaking coolant in.... how do I rule that one out?

Also, could vacuum leaks do the same?

 

[c1500deathtrap]

ok here we go;
1)that compression is excellent so we are mechanically sound
2)If i ever have to I usually install a gauge in the pillar permanently. Otherwise a really long hose t'd of of the throttle body input or something similarly sketchy.
3)Usually but i like to work in a systematic way eliminating potential issues.
4)pull all of your plugs and have a look you can google plug fouling charts to help you read them or post pictures of all 8 plugs up.If one plug is unusually clean compared to the others it can be coolant.
5)why yes, yes it can. but on a MAF controlled vehicle it usually misfires everywhere not just under load. unless its a vacuum leak that's ported I.E. it only opens under certain conditions like EGR.

The next best thing to do will be pull plugs and get pictures posted up. And check for vac leaks.

 

[DieselDetroit]

Good post. Very good post

We did have a vac leak at the brake booster (last year). I sealed it with rtv. I fished around with propane the other day but didn't find anything. I used a small hose with one side to my ear and didn't find anything. I may check again. It kept dying when she put it in drive before we fixed the broken booster with rtv. It makes sense. Could be vacuum.

See below for compression test results and plug results. Plugs are somewhat new but all looked the same.

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[c1500deathtrap]

Not to insult your intelligence but we need to see the shaded area of the insulator on the plugs but for the most part the plugs look alright so I rather doubt fuel is an issue... So the last thing i can think of is check the color of your spark it should be a nice bright blue to white when using a spark tester. Also disable EGR and run it under conditions that gave you a misfire. The last possibility that comes to mind is a bad cam position sensor or the dizzy is out of phase with the cam. Or its not a misfire at all! (would certainly explain the lack of codes)