'88 454 Backfiring Through Throttle Body

[clemsonjeep1]

this is about my 1988 R20 suburban with a TBI 454! I am having some issues with it running poorly right now and its pissing me off cuz I have a guy interested in buying it but it started running bad as soon as he got to my house the other day. I know some of you have some experience with GM TBI motors so I figured Id see if you could help me out. Heres whats going on:

 

It WAS idling poorly and backfiring through the TB...giving a check engine light...so I tried driving it around the block since it really hadnt been drivien all that much...when I got back to the house and popped the hood the passenger side exhaust manifold was glowing RED. I went ahead and replaced the wires, dist cap, and rotor.

 

Crank it up in PARK and it runs fine...you can rev it slow...rev it hard...no matter what it runs perfectly.

 

With it running and shift into DRIVE and begin to accelerate as soon as it begins to pickup speed and get a load put on the motor it backfires through the throttle body! I mean pretty little flames comin out of it it does not throw up a service engine light when this happens.

 

So at this point I have replaced: dist cap, rotor, wires and any vacuum lines that looked like they may cause a problem...I have pulled the codes off the computer from when it was running poorly and it gave me a code 33...which references MAP sensor circuit error (signal high indicating low vacuum)...but after working on it and tinkerin around and pulling some of the sensors out and running it without them to see if one is at fault...it gave me another code last night...34...which is the exact opposite of code 33. So at this point Im thinking that the MAP sensor is at fault but I really dont wanna spend too much seeing as how Im trying to sell this beast so whatever I do replace I would like it to solve the problem ...

 

I hope someone has an idea!

[fixitmann]

Its late im tired but if I had to through a guess i would say check vacuum for exhaust restriction or maybe torque converter issues. put vac gauge on it read at idle them hold steady at about 2500rpms see if it drops off. by the way what did plugs look like.

[minibike132]

Create an exhaust leak (disconnect ahead of the converter or remove o2s) and road test. Unless you have a backpressure gague. My first thought is plugged converter. A converter can cause a map code because the pressure is not something the ecm thinks it should see. If you breathe in and don't breathe out your chest will hurt cause it's not expecting you to keep breathing in...kinda the same thing. Might not be it but that's where I would start, that would account for a glowing manifold. After that look for a lean condition of some kind.

[bobcat5544]

I don't see how a clogged cat could cause a backfire issue.

 

Backfiring and glowing red exhaust manifold is consistant with a timing issue.

 

I would start off by checking timing chain slack. pull off the distributor cap and set the #1 piston to top dead center useing a braker bar and socket on the crank shaft make sure you turn it clockwise (do not rotate it counter clockwise). With it set at TDC mark the crankshaft and the timing indicater plate with whiteout. Now take the braker bar and rotate the crankshaft counter clockwise until you see the distributor rotor move. when you see it move a hair stop. now measure the difference on the mark you made if the marks are more than 1/8 of an inch then you have a streached timing chain replace timing chain set. If with in spec then go to step 2

 

step 2

Retime engine

 

If all above checks out fine let me know and we will go from there.

 

 

Codes 33 and 34 are setting due to the backfire

[Zembonez]

This sounds like a timing issue to me also. Have you checked the timing? Depending on miles, the timing chain may have enough slack in it that the timing is moving when running.

[clemsonjeep1]

i forgot to mention that i did check the timing...just hooked the timing light up...wasnt sure if i should disconnect anything...but it appears to be running somewhere between 4-6 according to the gauge on the timing cover...what should it be? i will check the slack as soon as it stops raining...

[bobcat5544]

The correct procedure to check/adjust timing is.

 

1. Have vehicle at normal operating temp

2. Air cleaner installed

3. A/C OFF

4. Transmission in neutral

5. Disconnect SET TIMING connector (tan/black wire) located in harness close to distributor. DO NOT disconnect 4 wire connector at distributor.

6. Connect timing light and adjust as necessary by loosening hold down bolt

 

Timing spec: 4 deg BEFORE TOP DEAD CENTER.

[jon99z71]

I had a 90 454SS pick-up that had the same problem, backfire through the TB and poor running condition. Turned out to be a cracked reluctor ring in the disrtibutor, it's the peace in the distributor that is pressed onto the shaft with 8 triangular shaped fingers on it with a magnetic pick-up coil. It's what trigers the ignition coil to fire. I'm not sure if you can press the old one off and press a new one on, without damaging one of the parts, so I put a used distributor shaft in that I had laying around, put it back together and it ran perfact.

 

 

I'm not saying this is your problem, but I went through a lot of diagnoses before finding that was the problem, including physicaly checking the timing chain.

 

 

Just a thought