olrac

Hello All,
I have a `97 Jeep Cherokee Sport 2.5 litre PETROL. The jeep is backfiring through the throttle body. After a while it clears but after start up the jeep lacks power and backfires and hesitates through the lower gears. It clears when in fourth and fifth. After sitting switched off for 20 to 30 minutes (still warm) the jeep will repeat this performance and needs a lot of throttle to clear through this phase. There is a severe lack of revs combined with popping and backfire at this stage.

Any help greatly appreciated.
Cheers,
David

SeriousOffroad

Sounds like either a valve timing issue (possibly jumped a tooth on the timing chain) or a bad PCM.

Both the 2.5 and 4.0L ignitions systems are virtualy identical, neither one has the option to mechanically advance or retard the distributor. Timing advance is handled by the PCM.

Jamie57

So the backfiring does go away correct?
And you're not getting a CEL (Check Engine Light)
If so I'd think some sensor is screwing up the ECM's ability to properly set the timing.
If it was a mechanical problem like a jumped tooth on the timing gear, it would continue to act up regardless of the engine temp or time running.
My first trick would be to reset the ECM so it uses the presets and let it relearn again.
It's a simple fast cheap fix and if it works great. If not we need more details and check the codes. That CEL may be burned out and it really is telling you to check some sensor somewhere.

olrac

Hello- replaced the engine 2 months ago and have swapped the sensors for new- MAP, TPS, AIC, O2 up and down, full tune and new O rings on injectors. Not replaced the injector rail as this looked fine. Fuel pressure is fine. Bought new ECM last week. ECM reset as well.

I can see the flame backfiring through throttle body when revving engine.

Driving me absolutely mad.....

Any help appreciated.
Cheers,
David

Jamie57

Can you answer that the backfiring goes away eventually?
It's an important question, so we can help diagnose your problem.
Another is, have you replaced your CAT? It sounds like you may be having too much back pressure. OR a burnt or bent intake valve. but those would happen ALL the time and not go away.

SeriousOffroad

When the engine was replaced did you properly index the distributor?

olrac

Hi -the CAT was replaced with a new one an also a SS exhaust was fitted. The problem does go away in higher gears but there is a lack of power in the higher range.
no idea about the indexing of the distributor. Can you explain that?
The old engine did exactly the same so when I replaced it I thought that I would be rid of the problem- not.

Cheers,

David

SeriousOffroad

This would have been good information to include in the first post!

Did you reuse the old distributor?

SeriousOffroad

The more I think about this the more i realize you've left out alot of important information. Diagnosing on the web is difficult at best and damn near impossible without all of the pertinet details.

What parts were replaced and what was reused?
Stock ignition system or modified?
Was a Leak down test ever performed to confirm that the injectors are not leaking?
Distributor properly installed (indexed)?

olrac

Hi All, Sorry for the ommittances but I have no idea what is pertinent or not. The engine replacement was the main block and we reused a lot of the original ancillaries. The engine swap was done not to eraddicate this problem -the old engine developed a hair line crack in the block so had to go. But as I said the symptons did not go away with the engine replacement. I started to replace sensors as the CEL was on and thrw codes that pointed to the TBS and the upstream co2. I have replaced the whole TB and installed new sensors there. The upstream/downstream CO2 sensors were done. New Cat, new tail end exhaust in SS.

Stock Ignition - no swap either.

Distributor was replaced with new cap and rotor- I did not install this but I understand it can only fit one way on the shaft- is that correct?

Fuel pressure on the rail has been measured and was fine.

The backfiring occurs at the throttle body when I have the air filter off to view this and when driving it oocurs in the 2nd and 3rd gear just after driving off -there is a distinct feeling of pressure building up on the throttle pedal, a great reluctance to pick up revs and a general feeling of something holding the throttle back -even although I am now flooring the pedal to try to accelerate through this phase. The CEL is not on. The revs are quite erratic at this pint and in town traffic there is a chance of stalling when trying to pull away at lights etc. I need to feather the clutch and really flatten the throttle pedal to get away. I release the clutch pdal slowly and manage to get through this. Once up to normal speeds the problem is gone but there is no real power and fuel consumption is horrendous - 15 MPG.

Hope this helps!
David

SeriousOffroad

I'd start by re-indexing the distributor, it won't cost you anything but a little time.

Remove the #1 Spark Plug
Remove the Dist.Cap
Remove the distributor.
Bring the #1 piston up to Top Dead Center on the Compression Stroke. Stuff your finger in the spark plug hole while someone bumps the starter. When yo feel air blowing by your finger STOP.
Use a 19mm socket and turn the harmonic balancer bolt clockwise (only) while keeping an eye on the timing marks on the timing chain cover.
Once the #1 piston is at TDC, re-install the distributor so that the rotor is pointing at the 5 O'clock position when the distributor is fully seated.

A word of caution.
Make sure that you rotate the oilpump shaft with a long screwdiver so that it lines up properly with the distributor. Failure to do so will prevent the distributor from fully seating.

Reinstall the cap
Reinstall the sparkplug
Double check the plugwires to make sure you have the correct sequence.
Fire it up.

olrac

Hi FrankZ -will try this - what is the logic behind this? I am not a mechanic so it would be good to know the physics behind the theory.
Many thanks,
David

SeriousOffroad

Easy.
The distributor timing is non-adjustable, therefore all ignition timing advance is handled by the PCM based on the Crank and Cam Position sensors. If the distributor is not properly installed the PCM cannot properly time the spark. It is very possible for a valve to be open when the corresponding sparkplug is energized.

RenoXJ

To make sure that the Timing in the DIST is synched with the timing on the cam / ecu.

kxe

If an exhaust valve insn't opening enough it will pop back into intake, pull valve cover and watch valves.