90 4.0 idle good buf backfire/die on gas

[scubasteve]

Hello,
I bought my first jeep today as a starter (to get around this winter and off road next summer). all i could afford for a toy for now... I am pretty mechannically inclined, (spart, plus fuel +air= go) etc, but I am not sure where to start with this project.

So with that said, it is a 90 "grand" cherokee laredo with a inline 6- 4.0 and if starts after a few seconds of cranking and runs pretty smooth at idle.
now for the bad. It will not accept the gas pedal being pushed. It starts rev up, but starts to backfire and pop and will not rev up at all. (let off the gas and it runs smoth at idle again... I think the engine has been replaced as the odo says 181k and the plugs are brand new and the guy I bought it off of said it was, but couldn't figure out the problem... so, in order to start my daig process, I thoguht I would ask any experts on here.


1. I herd that 89-90 have a different ignition/ fuel system that is more dificult to work on - any truth to this?

2. Does anyone know of an id tag/casting to tell the year of a block/ engine to verify engine is correct for the different system if applicable?

3. Where shoud I start checking other that fuel pressure/ air supply (lack there of -ie clogged cat/ filter) it apperas that the plugs are new, and the resister has been changed. It has a brand new radiator and had clean oil (ie doesn't appear off the bat to be head gasket...

I have read a bit on here already to see of similar problems and none seem to match my exact problem.

thanks
steve

 

[scubasteve]

update:
ok I figured out my first answer trying to connect obd scanner... RENIX, who knew.

second, I have good fuel pressure 35 at idle, 40 psi when I touch the throttle.

third, I checked the tps, I have .8 v @ idle, 4.2 @ WOT.
all those number are correct according to what I have read, anyone have any ideas how to set TDC on #1 for sure? and should the distributor be pointed at the cylinder? or the plug wire that goed to that cylinder?

thanks

 

[ashbranmeg]

Did you ever figure this problem out? I have a similar issue, and any input is welcomed.

Thanks

 

[scubasteve]

Did you ever figure this problem out? I have a similar issue, and any input is welcomed.

Thanks

Not yet,
I replaced the timing chain because it had some slack , the cps bacause I couldn't read it on the osc scope, the tps because the voltages were off.
Now it runs great it idle, but still pops and won't rev.
I started it before putting the drive belt asm back on and it reved up a little, but once the belt was back on the problem remained. I checked all the pulleys and they all spin freely.

I did fuel drop down on each injector, good fuel pres, plug gap checked. It is off time somehow.

So I redid tdc on #1 and set timing again and it still has problem.

Not sure what else to do except maybe computer or the cam lobes are bad not providing the lift?
Anyone know how to check that?

 

[ka041a]

my guess is that the spark plug wires are mixed up, or else the timing is way off because the only way you could get backfiring when the engine is cold is if a spark plug is firing when an exhaust valve is open and there is unburned air fuel mixture in the exhaust manifold. hope this helps, john

 

[ashbranmeg]

I solved my problem, and suspect yours is the same thing.

Haynes and Chilton manuals tell you to set initial timing on distributor by putting engine on TDC #1, then install distributor pointing at #1 on cap. This is WRONG.

It shout be installed exactly one tooth on the drive gear past this point. I moved mine one tooth forward and it runs great.

I have found many errors in these manuals over the years and am not surprised. You should try moving your distributor one tooth beyond #1 tower at TDC. MAKE SURE YOU MOVE IT THE RIGHT WAY, or the problem will get worse not better.

 

[scubasteve]

I solved my problem, and suspect yours is the same thing.

Haynes and Chilton manuals tell you to set initial timing on distributor by putting engine on TDC #1, then install distributor pointing at #1 on cap. This is WRONG.

It shout be installed exactly one tooth on the drive gear past this point. I moved mine one tooth forward and it runs great.

I have found many errors in these manuals over the years and am not surprised. You should try moving your distributor one tooth beyond #1 tower at TDC. MAKE SURE YOU MOVE IT THE RIGHT WAY, or the problem will get worse not better.

I will play with the timing some more, but I replaced the distributor thinking that the signal generator was bad and I still have the same problem... I am wondering if it could be electrical? It starts right up and will high idle when cold and pop as it gets close to 2000 rpm and drop to 1500 or so. Once it warms up and the idle slows, it is as smooth as can be....
but if I touch the gas at all, it starts popping at about 2000 rpm...

anyone want to buy a jeep