89 XJ 2.5 runs terrible

I recently acquired an 89 XJ with the 2.5 TBI 4 cyl. It ran good for the first 100 miles and then I stopped for gas one night and it hasnt been the same since.

It starts ok and idles ok. If you rev it up it will rev just fine all the way to 5500, but put it in gear (5spd) and go to pull out or accelerate and it bogs down, misses and backfires out the TB and exhaust. It bucks and jerks and barely gets to 30 mph.

Suspecting bad gas I empied the tank and refilled it. No help. I replaced the coil, plugs, rotor, cap and wires. No help. I set #1 to TDC and the timing mark is at 0 so I figure timing didnt jump? Im under the impression this thing doesnt store codes right?

Could it be fuel pump? TPS maybe. Im sort of at a loss here...Ive had a lot of 2.5s and 4.0s but they were all 97 and up.

It seems to be worse when its warm and it ran fine at least one complete day since this happened so Id say its somewhat intermittent.

I did a compression check and got 100 psi on 3 cyls and 90 psi on the other one. Not great but it does have 125,000 miles on it and a lot of blow by.

One last thing....could it be the ignition module behind the coil?

Check your fuel pressure? I'd guess ignition system but you already replaced most of the common stuff. Check how hot your spark is?

EDIT: also, you can try putting a little (VERY little) oil in the cylinders with the low compression, then check again and see if that fixes it temporarily. Should find out if the rings are causing the leakage vs the valves.

I have the same engine and had similar problem. Mine was the fuel filter...

kastein said:

Check your fuel pressure? I'd guess ignition system but you already replaced most of the common stuff. Check how hot your spark is?

EDIT: also, you can try putting a little (VERY little) oil in the cylinders with the low compression, then check again and see if that fixes it temporarily. Should find out if the rings are causing the leakage vs the valves.

Click to expand...

I did the oil trick and it helped, im not thinking the compression is causing it though. Whats a good way to check for good spark. Im trying to find a good way to tee the fuel press gauge in. I was thinkin timing but wouldnt that throw valves off and cause 0 compression?

xjintx said:

I have the same engine and had similar problem. Mine was the fuel filter...

Click to expand...

I forgot to add I changed the filter twice......thanks though.

A fuel pressure gauge should connect to the fuel rail test port pretty easily, just attach it to the schrader valve.

As for testing spark - pull a plug wire, put a spare plug in it, wrap a piece of wire around the threads, clamp with a hose clamp, connect other end of wire to the block/AC compressor bracket/whatever good ground you can find. Have a helper turn the engine over (while it's in park/neutral of course) and see if you get a bright blue snapping spark or a weak yellow spark.

It doesnt have a test port (I wouldnt be that lucky). I did the spark test. It looks good but doesnt really snap like some others Ive seen, but I replaced the coil, what else can cause a weak spark.

How's the braided ground strap that attaches to the driver side firewall? Remove the 15mm bolt and scrape the paint off underneath an dmake the ring shiny. Also check it at the head. On another note, we had one like yours at the dealership and it was a few years old. The guts of the catalytic converter plugged the muffler.

Although some people have posted that they have 4L's that run great with less than 100PSI compression, that is super-borderline. I have personally seen a 2.5 - 4banger Fiero that wouldn't start in the winter due to 100PSI compression. Re-ringing/hone was required to get it to start. Is this Fiero Iron Duke motor not the same as the old 2.5 XJ?

I've got a similar problem with my 2.5. Mine goes away after a mile or two though.

Might also look into a plugged catalytic converter. I had a 4 liter that wouldn't do 60 mph in 4th gear that had that problem.

cruiser54 said:

How's the braided ground strap that attaches to the driver side firewall? Remove the 15mm bolt and scrape the paint off underneath an dmake the ring shiny. Also check it at the head. On another note, we had one like yours at the dealership and it was a few years old. The guts of the catalytic converter plugged the muffler.

Click to expand...

Grounds look good, going to drop the exhaust this weekend and see what that does.

winterbeater said:

Although some people have posted that they have 4L's that run great with less than 100PSI compression, that is super-borderline. I have personally seen a 2.5 - 4banger Fiero that wouldn't start in the winter due to 100PSI compression. Re-ringing/hone was required to get it to start. Is this Fiero Iron Duke motor not the same as the old 2.5 XJ?

Click to expand...

2.5 XJ is an AMC engine not the iron duke, but I'm beginning to wonder if maybe the compression isn't the problem, but it starts and idles and runs fine, you just can't put any kind of a load on it. It really seems like the ignition system, but Im gonna drop the exhaust this weekend and rig up something to test fuel pressure.

Starts and idles fine, but can't handle a load I would be suspect of the ignition or fuel.

Hook up a vacuum gauge and attempt to slowly increase the amount of throttle. If the vacuum drops off then the exhaust may be plugged.

If you raise the throttle too quickly the vacuum will drop suddenly, so only increase the throttle slowly.