'98 Tahoe 5.7 Vortec runs like crap, backfires in exhaust and intake - need help, pls

[Tempritscher]

Hi,

I am Markus from Northrhine-Westphalia, Germany.
Here in the "old world", trucks are not common like in North America,
so there are no GM garages here and no experts for these type of cars :-(.
I've got a friend who drives a '88 GMC Sierra, but he has not a vortec engine with
CSFI, like i had. So he doesn't really know how to help me, since I replaced some parts
at my truck, including to convert from the CSFI to the MPFI...

Engine coolant temperature, intake temperature, mass air flow and manifold absolute pressure
seem to be okay for me (using torque app for android for checking OBDII).

I have a 1998 Chevy Tahoe 5.7 Vortec, 4 speed auto

Here is the story:

A few weeks ago, maybe four or five, my engine began to run poorer, starting with sometimes bad idle at the traffic lights.
One time I stopped at a gas station, I touched a spark plug wire and got an electric shock,
so the first things I replaced were the spark plug wires, the distributor cap and the distributor finger, but no improvement.
Engine still ran creeping poorer than a month ago.
Next I replaced the MAP-Sensor, the new one works great I can see via Torque app for android.
But still no improvement.
Anytime the MIL started to flash and i got misfire in the exhaust and in the intake manifold.

By googling, I found out that many guys have converted from the CSFI I had to MPFI
because the CSFI is deemed to be unreliable.
So I orderer the MPFI from rockauto, and a week later it was here in Germany.
When unmounting the upper intake manifold, saw much carbon debris in the front,
where the nozzles for the cylinders 1, 3 and 2, 4 are positioned, backside with
5, 7 and espacially 6, 8 were only little debris.
After cleaning and installing of the MPFI, still no improvement.

Still flashing MIL, backfire in the exhaust.
The worst engine run with backfires is under load an lower rpm,
higher rpm and less load is better.

So after the MPFI conversion with the still existing problem,
I changed all eight spark plugs (old from AC Delco, new from NGK),
but still now improvement in engine run, and still backfires

Please give my an idea how to search further...

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[oohunter04oo]

You need to check your timing. You would need a scan tool like a tech 2 to read the timing data.

 

[Parentnoia]

This sounds like the exact same issues I'm having, except that I havent thrown money at it yet, just time.

 

[94Sierra4x4]

Your new plugs are not gapped right as far as I can tell. They look like they are gapped at 0.035" and not the recommended .060"

Check that first Unless you regapped after that pic.

 

[Tempritscher]

@94Sierra4x4:
My repair manual says spark plug gap has to be 0.045" (1.1 mm). The new plugs are gapped this way.

@oohunter04oo:
I would like to check the cam retard, but here the local Opel dealer hasn't got a memory card for the GM cars from North America for his Tech II Scanner :-(.
Thinking about buying ScanXL Windows OBDII scan tool + GM Package + OBDII Adapter. But will cost more than €300 alltogether.
You mean my truck got a really bad ignition because maybe the cam retard got creeping worse?

Couldn't be the camshaft gone bad or so? Hydraulic lifters made some noise on cold start since I bought the truck last fall (used engine flush and changed oil after purchase - thought of doing a good thing, cleaning oil channels and so).

But did a compression test on monday, seemed to be all cylinders had compression and no one with noticable less compression.
highest was 8.5 bar, lowest 7.8 bar / 123 psi to 113 psi.

Anyone tips? This truck is so uncommon here and no one seems to has the right clue here in germany near me.... ;-(

 

[heavychevy98]

Changed plugs wires and all and now backfires? Check and make sure your wired it back right. A pair of plug wires in the wrong place would make a cylinder fire when the valves are open thus a pop or backfire

 

[1997chevydriver]

The carbon build up on the front half of the intake manifold is normal. That tube that sticks up right in the middle is where exhaust gasses enter from the egr valve.

As for missfires/ backfires. Are you able to have your ignition module tested? Its on the same bracket as your coil

 

[1997chevydriver]

Also if your spark plugs were in that poor of condition, did you look at the inside of the dizzy cap and the rotor?

 

[94Sierra4x4]

I still think that plug gap isn't right. Your old plugs looked to be at gapped bigger than the new ones.
I agree with heavychevy, check your spark plug wires for proper routing, make sure ya didn't cross a couple wires.\

And Paul, he replaced plugs, wires, dizzy cap & rotor.

 

[Parentnoia]

I dont know what repair manual you're referring to, but my Haynes manual says .060 and so did a few other folks here, so it might be in your best interest to at least try gapping them to .060