Suburban 454 Issues

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Good evening ladies and gentlemen,

I've been very busy lately transitioning from active duty USN to the reserves... I have to say, the "freedom" is nice, but I'm getting bored! (Un)fortunately, my 1996 Suburban 454 has been keeping me busy.

No CEL, no codes stored.

Once warmed up/in closed loop, there is a significant stumble between idle to 1.5/2k rpm when I give light throttle or when under load. If I give it a significant amount of gas, there seems to be no issue. Once up to speed, there is a slight hiccup on occasion, but again, giving it "more gas" clears up the issue.

Work completed recently (not necessarily related to this issue):

Fuel pump
Fuel filter
Distributor, cap, wires, plugs
All 4 O2 sensors (thanks GM)
MAF sensor

Symptom elaboration-

When I unplug the MAF sensor, which I believe triggers open-loop operation, the problem completely clears up. Runs great, but likely gets terrible mileage, triggers the CEL, and won't pass emissions.

The PCV passes the vibration/wiggle test.


What I think it could be:
Vacuum leak? I sprayed carb cleaner around the few vac lines I could identify, but noted no change in RPMs. Certainly doesn't mean there isn't one.

Intake manifold gasket leak? Unplugging the MAF could be allowing the engine to better compensate for a possible gasket/vacuum leak.


Any suggestions/recommendations would be awesome! Thank you
 
Originally Posted By: HerrStig
Crank position sensor?

or throttle position switch (since giving it more gas moves the position of that switch and the problem stops)?
 
Problem goes away with MAF disconnected... I would think the CPS or throttle position sensor would cause issues regardless? This thing is giving me grey hairs!
 
I've had a situation where I had to exchange 2 MAF's to get one that worked correctly. Especially if you are using an Autozone or similar rebuilt unit - get a new one. You did the best diagnostic when you unplugged the MAF and made the system use the MAP sensor.

REplace the MAF.

Also - you only had to replace the 2 O2 sensors before the cats because the 2 rear ones do NOTHING to your engine management. They just monitor the cat converters. Everyone, not just GM, has 2 O2 sensors per bank depending on the exhaust system.
 
These engine were known to have bad injectors as they age, the OE Multecs are very trouble prone and can cause symptoms like the ones you describe.
 
I'd clean the throttle body and make sure that doesn't have a carbon build up in it.

Then I'd look at the air filter and fuel injectors.
 
MAF and Injectors. You will be good to go!

Welcome to Freedom brother! The boredom will end in about a year.
 
It might be the crank sensor. If it's anything like my 96 Chevy truck with a 350 then the crank sensor is really easy to change. 5 minutes tops. It just isn't cheap.

What brand of distributor cap and rotor did you use? I believe they're really picky and really like AC Delco caps and rotors. It also could need a distributor.
 
Try 2 bottles of techron in the gas tank before you fill the tank then clean the throttle body very good with a throttle body cleaner and see if that helps first..Sometimes the simple things are the things that work.
 
Ok my game plan for tomorrow:

1. Another MAF
2. Clean throttle body
3. Replace injectors

Hopefully any one of those will work and I wont have to continue down my list any further. I'll report back with results. If anyone has tips or recommendations, I'm all ears. Or all eyes?

Thanks guys

PS I used ac Delco replacement distributor.
 
Originally Posted By: BowNisPar
Saved round:

Would injectors still be a likely culprit if it runs great with the MAF unplugged?


+1

If you start parts throwing, it gets expensive real quick. Proper diagnosis is paramount.
 
Originally Posted By: Brybo86
Injectors aren't cheap I would do the MAF and throttle body cleaning then 2 tanks of techron before replacing all 8 injectors...


+1 if that doesn't work shoot Trav a PM about cleaning the injectors. He hooked up a lot of members here with good prices and service.
 
Those injectors don't clean well Frank, i don't like doing them. Those and the Magneti Marelli have a very high failure rate.
Replacement with the Bosch 3 style is the only way to go. They are plug and play and flow 19lb just like the originals but with much better atomization and superior internals.
 
Update:

Another new MAF, no change.
Cleaned throttle body, no change. It was pretty clean (very, actually).

Are we still thinking injectors? I priced them out...not cheap!

Finally, this morning the symptoms started immediately, engine cold. However, when I unplug this new MAF, it still runs great again, albeit with a CEL... Now I'm pulling out my grey hairs!

Edit/addition: When the engine stumbles/skips, there is a chirping sound from the engine.
 
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Let's see, my knowledge is elementary.. The MAF compensates the amount of the air based on atmospheric conditions. No MAF and you're running fine. With MAF not running fine. What does the MAF control? It fine tunes to tell your car's computer how much fuel to deliver. You're either getting too much or too little fuel (rich/lean) based on the signal from the MAF. Either your MAF is wacky, the computer is sending wrong signals, or the fuel delivery is not what the computer thinks it is.

Can you tell if you're running rich or lean?
 
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