Hey guys, I am about wore the hell out on trying to figure out what is going on with this car (1992 5.7 tpi). It all started when I stomped on the throttle to beat a red light and it immediately started backfiring. Its got a new distributor on it with timing re set to 6 degrees BTDC and the timing will jump up and down when connected to timing light. we checked eveything with the scanner and my O2 is reading 10K ohms, map at 800v, etc. replaced the O2 and MAP sensor and still no dice. can yall think of anything?

I thought of the timing chain being jumped but it seems to run too smooth for that on idle. just under acceleration it bogs down and misses/backfires

 

Stretched timing chain? My truck acted exact and it needed a new chain

 

Did you check for any vacuum leaks?? Especially around the throttle body and intake base

Stretched timing chain is very possible due to it happening after you stomped on the gas pedal or it has jumped a tooth which throws off timing??

Distributor could have also jumped a tooth or its gear could have chipped/broke where it meets the cam??

 

Has the balancer outer ring slipped?

 

Sounds like it is in the dizzy could have lost a spring on the advance weights or the vac diaphragm could be bad if it has one etc.

 

IHave you checked the TPS voltage? For idle voltage, WOT voltage, and smooth transition between the two as the throttle arm is moved?

You really can't check the timing chain position with a timing light. But to check the timing you need to disconnect the EST connector. Also recommend using a dial back timing light so you can verify the timing is correct at higher RPM.

 

It's a computer controlled distributor. No weights or springs or vacuum canister.

 

I recently replaced the intake gaskets about a couple months ago and in the process i replaced all of the vacuum lines so I don't believe that could be an issue.

 

Im going to get those reading to you tomorrow, its supposed to be .54 at idle correct? stretched timing chain was my first guess but id like to make sure before i tear down the front of the engine.

 

Yeah .54 at idle and between 4.5 and 5 at WOT.

 

On speed density you can go .76 on the tps setting.

 

IIRC, the 90-92 SDs use a set it and forget it non adjustable tps from the factory. As in whatever voltage the ecm sees at key on or off?, that's what it uses for the base tps setting. (as long as its normal ranges)
Unless I misread the FSM ?

 

The tps is non adjustable but adjustable tps are cheaper for some reason and will screw right on.

 

Correct. The concern is mostly smooth transition from idle voltage to WOT voltage.

 

Ah yes, very important

 

Also What is the BLM at idle? A lean condition is the most likely culprit here IMO...

 

alright so I haven't been able to use the scaner tool lately to read all of the sensors again, however I did check the slack of the timing chain and it seems to be 6-7 degrees of slack.