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OT...5.7 Chevy misfires

S

Stockdale Dave

Member
I've got a 97 Tahoe with the 5.7 350 @192K miles. Started running rough and when we put it on the scope it shows constant 5 and 7 cylinder misfires. Fresh AC DElco plugs, cap, rotor and wires and it still does it. Compression check is good on both cylinders. I am stumped. 2 mechanics are stumped. Any thoughts guys? Oh yeah OBDII codes also show left bank rich condition.
 
Pretty sure this one has the single coil configuration. Thanks for thinking about it though...this is driving me nuts. My gas mileage has dropped to 15 MPG from 17 so I need to get this working on all 8.
 
My 1996 blazer did the same thing Gm dealer said it had to be machanical and wanted to replace engine He had already replaced fuel pump and cleaned injectors with his labor and parts cost me $1500 and it still didn"t run rite and flashed codes . Went to an independent shop they replaced cam sensor and it worked for 2 years Then it started again that time we replaced the coil and it is still working. I would start with the coil then cam sensor if it was mine . I hope this helps MIKE
 
Had similar one here in shop last week , 2 bad injectors . Fixed it right up . Not sure if thats what you have or not,,,but tossing nother idea out there .
 
I'm getting old and can't remember much anymore, but some ideas:

What kind of compression readings are you getting? Chevies have trouble with anything less than about 125 lbs of wind.

If the compression readings are absolutely and totally "identical", might suspect a head gasket, as commonality on those two cylinders, as you already know, are the firing order and their physical location.

I'd ignore the "rich condition" code; just means it isn't hitting on half of the left bank.

Guess I'd probaly pull that left rocker cover and see if you're dealing with a flat cam or broken valve spring(s). Valves that aren't opening can still throw a compression reading that can mislead you.

Didn't happen to cross those two wires in the firing order, did ya? Does this one have a distributor? Wobbled out shaft?

Wobbled out guides can just drive ya nuts to find. Compression guage will usually "blink" a few times before it finally takes a reading.

Keep at it; there is a reason there somewhere.

Let us know what you find, will ya?

Allan
 
Have someone do an injector fuel pressure drop test on the injectors.Those 5.7(R's) are prone to clogged poppet valves at ends of injectors.

Basically,with engine off,pressurize the fuel system and activate an injector on for the duration of the test to see if any injectors drop significantly less than others.A tech 2 scanner has the test in it's software.Not sure about other scanners.

May have to take to dealer to have them run the test.Takes about 15 minutes.
 
If any injectors in that "octopus" need replacement, GM has a kit to update the whole injection system for little more then a couple of injectors cost.

There is a TSB on it.

I would think a failed intake gasket could possibly be the culprit, as well.
 
It has a leaking fuel pressure regulator or bad head or intake gaskets.  Misfiring cylinders will cause the computer to show that bank being rich.
 
(quoted from post at 14:55:04 07/19/08) I'm getting old and can't remember much anymore, but some ideas:
......

Didn't happen to cross those two wires in the firing order, did ya? Does this one have a distributor? ......

Let us know what you find, will ya?

Allan
Click to expand...

On those engines the #5 and # 7 plug wires must not run next to each other or they can cause a misfire. When coming from the distributor run #s 1 and 3 between 5 and 7.
 
Odd that you would say fuel pressure regulator. I had a GM Master tech email me to say that he had seen this issue and it was a leaking regulator under the intake plenum. It did start shortly after I replaced the fuel pump. Just can't see how a leaking pressure regulator would cause a 5 and 7 misfire consistently though. I'm going to pull the valve covers and look for unusual spring action and pull a compression test before I pull the intake. Thanks for your input.
 
Regulator is inside intake Plemun . They can leak externally , causinr raw fuel to run into closest intake runners ,,possibly 5 & 7 . Do plugs on those cylinders look wet, or fouled ?
 
Remove the upper half of the intake plenum. With it out of the way , you can cycle key on ,off torun fuel pump and pressurize system. You should be able to see any leaks then . Keep us posted
 
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