96 Tahoe Engine miss at idle

69ARROWHEAD · October 11, 2005

I have a 96 Tahoe and it is experiencing a noticable miss at idle. Sometimes it is hard to start when warm and there is noticable black soot at the tail pipe. Not smoke just black sooty residue.

I have changed the plugs, wires, cap, rotor, fuel filter all twice and the miss still persists.

Earlier in the year I had a service engine soon light and decoded as egr valve. I let this go for a while and finally replaced the egr valve. I then checked for codes and now had oxy sensors, EGR valve, carbon in intake tract, and multi cyl miss fire. I took the egr valve off and installed the old one, cleaned the intake with seafoam, removed and cleaned oxygen sensors, and ran it for a short while. So all carbon could be cleaned. I then replaced the egr valve again with the new one, cleaned intake again and now all codes clear except for 300 - multi cylinder miss fire.

I removed all 8 plugs and the #5-#8 (back four cyl) show noticable black soot that cleans off easily. Oxygen sensors again sooty and continued miss at idle. I removed the oxygen sensors located before the cats and started the Tahoe and could not notice a significant change in idle.

I was told the fuel regulator may be the culprit but this does not make sense to me. I would like to check the fuel pressure before I dive into that. What should the PSI be and where should I take a reading?

Are my suspicions correct in a partialy plugged catalytic converter(s)?

Tahoe runs good on highway. It only has 56,000 miles on it and is pampered.

Any help appreciated, Alfred

96 Tahoe 5.7 L 2wd auto

GM-Tech · October 22, 2005

No special instructions or tools needed, that I can think of. Shouldn't take more than an hour or so.

GM-Tech · October 18, 2005

When you give it throttle it drops (pressure) slightly also. Should this happen?

Vacuum drops momentarily when you open the throttle. Lower vacuum should result in the fuel pressure regulator INCREASING fuel pressure, not having it drop.

05HD · October 14, 2005

As GM Tech mentioned earlier, watch for the fuel pressure to drop off with the engine off. Pressure should stay up (exact number isn't important but should stay right where it stops when the pump shuts off) for quite a while after the engine is off. If it is dropping off, it doesn't have to be the regulator though, may be a leaking check valve in the fuel pump or another leak pre-regulator. I like to pull the brake booster hose fitting off of the upper plenum and peek inside for a "washed down" area on the lower plenum, indicating a fuel leak in the manifold. Your earlier decarbonizing efforts may make this quick check invalid though.

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I dont have a way of checking that on my truck because the braking on my truck dosent have the big round booster like the older ones do mine has the elec power looking one.I can get a pic if needed.

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You have a big block, correct? Completely different than the small block in the other guys truck.

69ARROWHEAD · October 13, 2005

No reply?

I just did a vacuum check at the pcv inlet (to the engine) and the engine is pulling a solid 20" of vacuum. I did the entire top of the engine check for any vacuum leaks and can't find any. It certainly looks to be a lean condition because of the black soot.

I made a fuel pressure gauge and mounted it to the top rail shrader valve. It read a near steady 50+ psi. Granted this was a rigged up gauge.

I was told to check the ignition coil but no auto parts in my area can check it. I don't see any arcing of the ignition system at night.

What am I missing (pun)?

Desperatly needing help!

Alfred

96 Tahoe 5.7L 2WD Auto

69ARROWHEAD · October 19, 2005

When you give it throttle it drops (pressure) slightly also. Should this happen?

Vacuum drops momentarily when you open the throttle. Lower vacuum should result in the fuel pressure regulator INCREASING fuel pressure, not having it drop.

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CORRECTION, fuel pressure does increse with throttle. It goes up by about 5-8Lbs.

I checked the fuel pressure again tonight with a more accurate fuel pressure gauge and fuel pressure read 65 lbs with key in the "on" possition but then the pressure drops almost imediately after the pump stops. This with truck not running and key to the on position. I start the truck and fuel pressure reads about 55-58 lbs and increases with throttle as it should.

So, the pressure should stay at 65lbs after the fuel pump is activated, correct?

Recap: truck runs good on the highway, good power at speed but I get a slight hesitation at take off and it acts like it chugs as I come to a stop the last couple of feet as it rolls to a stop.

Engine has a very noticeable shake in Park (idling).

Sorry for the confusion. Alfred

GM-Tech · October 13, 2005

A catalytic converter that is restricted typically will not cause a miss at idle then run fine down the road. It's usually just the opposite. Clogged cats will often allow the vehicle to idle like there's nothing wrong, but the moment you try to move, you notice the definite lack of power.

Soot is not lean, it's rich. Why is it rich? That I can't answer. How exactly did you clean the o2 sensors?

69ARROWHEAD · October 13, 2005

I removed the two oxygen sensors before the cat and they had soot covering them. I sprayed GM non chlorinated parts cleaner on them.

Running rich, so fuel is not being burned completly. So that brings me back to the ignition? How about ignition coil or module? As I said no parts place can test mine. How can I test? I have a multi meter.

Just to let you know, my coil has a single 4 prong weather connctor. I had a friend lend me a known good coil but the connector is different. It has the 2 seperated weather connectors (still 4 prong but two each). It looks similar everywhere else.

Thank you for the input.

Alfred

GM-Tech · October 13, 2005

I don't see an ignition coil or module causing a problem just at idle, or just a few cylinders running rich. We have no way to test a module in the dealership either. Replace it and if that fixes the problem, it must have been the module. Crappy way of diagnosing it? Yeah, but it's what we got. Somewhere there are probably resistance values for the coil. In my experience, the resistance could be good and the coil is still a problem. So that's a pretty worthless test too. But as I said, this sure doesn't sound like coil/module in any way, shape or form.

My first step would be to put a pressure gauge on the system, run the pump to pressurize it, then watch for the gauge to fall off when the pump is not running. This could be due to a leaking fuel pressure regulator. However, the fact that the pressure does not fall off when the engine is not running doesn't mean it couldn't be leaking when it's running.

My next step would probably be to do an injector balance test. You do this by running the pump, fire 1 injector, note the amount of pressure drop. Do this for all the injectors and they should all have relatively the same amount of pressure drop. This is not a test you could do at home without the equipment.

GM-Tech · October 19, 2005

Pressures sound good. When the pump shuts off, pressure will drop 5 psi or so, but it should hold there for a bit. It won't stay at, say, 58 psi forever, but it shouldn't drop like a rock to zero either. Besides, if pressure is good while it's running, a leaky check valve in the pump would have no effect on the engine when it's running. It would only contribute to a hard start.

swilliams1005 · October 13, 2005

I don't see an ignition coil or module causing a problem just at idle, or just a few cylinders running rich. We have no way to test a module in the dealership either. Replace it and if that fixes the problem, it must have been the module. Crappy way of diagnosing it? Yeah, but it's what we got. Somewhere there are probably resistance values for the coil. In my experience, the resistance could be good and the coil is still a problem. So that's a pretty worthless test too. But as I said, this sure doesn't sound like coil/module in any way, shape or form.

My first step would be to put a pressure gauge on the system, run the pump to pressurize it, then watch for the gauge to fall off when the pump is not running. This could be due to a leaking fuel pressure regulator. However, the fact that the pressure does not fall off when the engine is not running doesn't mean it couldn't be leaking when it's running.

My next step would probably be to do an injector balance test. You do this by running the pump, fire 1 injector, note the amount of pressure drop. Do this for all the injectors and they should all have relatively the same amount of pressure drop. This is not a test you could do at home without the equipment.

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My dealer did the injector test and the number 2 and 6 were flowing more fuel than the rest however we swapped injectors from side to side and those same to are still doing the same thing so its wasnt the injectors so im not sure where to go now other than replacing my pc,, GM-Tech and suggestions?

69ARROWHEAD · October 19, 2005

Fuel pressure drops significantly to about 20psi or so after a minute with key in "on" position. So does that mean this is just the fuel pump check valve or the actual fuel pressure regulator at the back of the engine? Would the regulator cause the bad idle condition I'm experiencing? Leaking internally?

Any other simple checks for me to look at?

I still have the ignition coil I'd like to check. Service manual says a bad coil causes miss or "poor performance" if I remember correctly. I've read so much so fast. Want to get this taken care of.

Alfred

GM-Tech · October 13, 2005

My dealer did the injector test and the number 2 and 6 were flowing more fuel than the rest however we swapped injectors from side to side and those same to are still doing the same thing so its wasnt the injectors so im not sure where to go now other than replacing my pc,, GM-Tech and suggestions?

You kind of came out of left field with that... care to share what problem it is you're having?

PCM's rarely have these kinds of problems... or any problems really. That'd be the last place I'd look.

05HD · October 14, 2005

How about the possibility of a leaking fuel pressure regulator? We are seeing these pretty regularly along with stuck poppet nozzles (on CSFI and CFI systems) with the poor fuel quality here in the northeast.

69ARROWHEAD · October 14, 2005

How about the possibility of a leaking fuel pressure regulator? We are seeing these pretty regularly along with stuck poppet nozzles (on CSFI and CFI systems) with the poor fuel quality here in the northeast.

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Your the second one to suggest the regulator. And my source also mensioned the poppet nozzels. I would like to check the pressure again. Is there a way to tell if the regulator is leaking? What should the pressure be and should it stay at this pressure and for how long? Is swapping out the regulator difficult? I know I'll have to remove the upper plenum. I'd like to do it this weekend. Any insight would be helpful.

Thanks, Alfred

69ARROWHEAD · October 31, 2005

!SUCCESS! Finally got a chance to tear into the upper intake plenum to replace the fuel pressure regulator. Upon removeal of the plenum, guess what I saw? The regulator was obiously leaking onto the back half of the engine and the back part of the intake was completly "washed" and this explained the fouled plugs on cyl 5-8. I ran my fiinger under the regulator nipple and it was dripping fuel.

It did take me a little longer than expected. I had it all buttoned up and started it. No more miss but had a bad vacuum leak around the injector harness plug and fuel inlet. I gambled on a maginal o-ring and it failed upon reassembly. Took it completly apart again with a new o-ring and started it again.....

Truck purrs like a kitten and runs like a top! I can't even tell it's on at a stop light!

NO MORE MISS!