smooth idle... cruise... rough under load

what might cause a 4.0 (1990 MJ with 100K miles) to run perfectly smooth at idle and at any other RPM, but when accelerating it has what feels like a misfire. It's rough and is there at any RPM while under load. The roughness is direct with engine RPM so I've ruled out anything behind the transmission input. It does this in any gear (5-speed AX-15) and simply feels like a dead cylinder.

I just finished up swapping in everything from the engine back from my XJ and went to take it for a drive and now the engine is uke:.

The only things I've done that are engine related since it ran perfectly 3 weeks ago is install a K&N filter... connected the large valve cover vent to the canister vent to the intake tube right behind the filter...and replaced all the EGR vac lines with rubber. Unplugging the EGR has no effect on this issue.

The biggest gripe is that the thing idles smoothly and I can't seem to replicate the misfire by revving the engine... there has to be a load on it. This makes it difficult to fiddle with. Luckily, for once this isn't an intermittent issue.

plugs, wires, distributor cap, rotor, IAC, TPS, CPS, coil, ignition box under coil, and ECU have all been recently replaced or swapped with others to test... no improvement.

I'm leaning towards a clogged injector, or mechanical damage to the valves, rings, etc. I'm about to perform a compression test to rule these out (hopefully). Pulling the injector plugs one-by-one results in a rough idle... one-by-one.

What do you all think?

Thanks

I have also swapped O2 sensors to no improvement.

Anyone?... I figure the distinguishing symptom is that this missing/vibration is only there while the engine sees loading. Could it be vacuum related? That's also related to load. Do you think a bad MAP could do this?

Thanks

Under load misfires--crappy spark plug(s), cap, rotor, wires, or failing fuel pump. Start with those.

I'll swap them again tonight just for giggles... (im praying it will fix this). What about injectors? Could i pull the wires off one-by-one and go for a drive that way to test? Do you think it would damage anything?

Thanks

Most likely a spark issue. When under load, there is more resistance of spark to jump the gap in the plugs, so sometimes it goes to the next weakest link. I had this issue on my Crown Vic whan it was wet. Dielectric grease in the plug boots fixed it. Could it be moisture in the dist. cap or something like that? Sometimes you can see carbon tracks in the cap or on the rotor. It doesn't seem like it could be an injector, but you could briefly test them like you think.

jjvande said:

I'll swap them again tonight just for giggles... (im praying it will fix this). What about injectors? Could i pull the wires off one-by-one and go for a drive that way to test? Do you think it would damage anything?

Thanks

Click to expand...

should be able to see which cylinder is missing by the condition/color of the spark plugs. a miss is fuel or spark related. no spark and a plug will be soaked with gas, no fuel and plug will be white and chalky from a lean condition.

under load conditions, i would look at fuel pressure, sprk plugs, cap, rotor, wires and posibly the sync generator in the dizzy that controls injector timing.

before that, recheck the work you did before all this started. the rubber lines for the egr, are they vacuum hose line? egr is under vacuum and you may be collapsing the egr vacuum lines if there not designed for vacuum. at idle the egr is closed so i beleive at idle, it wont have effect on idle if you disconnect. egr partially open under load may be a problem.

89xj said:

should be able to see which cylinder is missing by the condition/color of the spark plugs. a miss is fuel or spark related. no spark and a plug will be soaked with gas, no fuel and plug will be white and chalky from a lean condition.

under load conditions, i would look at fuel pressure, sprk plugs, cap, rotor, wires and posibly the sync generator in the dizzy that controls injector timing.

before that, recheck the work you did before all this started. the rubber lines for the egr, are they vacuum hose line? egr is under vacuum and you may be collapsing the egr vacuum lines if there not designed for vacuum. at idle the egr is closed so i beleive at idle, it wont have effect on idle if you disconnect. egr partially open under load may be a problem.

Click to expand...

Thanks,

posibly the sync generator in the dizzy that controls injector timing.

Click to expand...

what is this ? haha.

distributor= dizzy?

ok, im going to go work on it now.

sync in dizzy = Cam position sensor in the distributor.

mine was doing the very same thing yesterday as well.......

anyways i too just changed the plugs etc etc, but i put a new fuel pump in today and it seemed to take care of the problem.

I just went through this same issue two weeks ago and it was really frustrating. I had a faulty MAP sensor - $40 at Pep Boys. I noticed that you haven't checked this. Just replace yours if you haven't done so yet. Hopefully this will be your problem as well.

Last night I got a few hours to work on the jeep.

I replaced the cap, rotor, wires, plugs, vacuum lines, and battery cables. I also did a compression test (~120 on all †6) and checked the distributor.

None of this fixed it, but it's good to get done. The jeep thanks me.

Tomorrow I'll get a chance to replace the MAP and check the fuel side.

Thanks

MAP didn't fix it.

I went and got one of those mechanics stethoscopes from harbor ($4) and took a listen.

EVERYBODY should own one of these!

you can hear everything inside the engine working, or not, in the case of my jeep.

The sound that i believe is what I'm hearing, feeling, and calling a misfire... i hear only when i blip the throttle. I pinned the tip of the scope on each injector and noted each one was clicking and at the same rate/response to throttle blip. The misfire sound is like a dead thud/pop type noise and the sound increases in volume the closer you get to the rear cylinder. The rear two cylinders produced the same level of the noise in question.

Like I said, i can hear those two injectors clicking just like the others... but also this other noise in there.

I took all the injectors out to check the tips and can't see anything special (they're ford 19lb from the 5.0). They all look the same... little bits of deposits on them, but not what I'd call dirty. All the areas around the orifices (4 on each injector) are perfectly clean.

I'm going to reverse the order front to back to see if the noise follows the injectors.

The noise stayed at the rear cylinders after swapping around the injectors front to back.

Could just the rear of the rail be starving if fuel pressure was low? I'm pretty sure it's only the rear cylinders acting up. Or, is this noise the sound of a sticking valve that the low speed compression test didn't reveal?

We did have the flywheel off during the swap of all these parts from my 88 xj. Could we have messed something up in there to throw off the timing or something?

Thanks

Use a vacuum gauge.

Watch for irregular drops in the vacuum--sticking valves.

Fuel pressure test shows about 27.5 at idle, unplugging the regulator yielded about 35 or so. A blip of the throttle and it would jump up to about 30.

I'm going to get a longer line and go for a drive....

decided to do some more fuel testing. I ran the motor... let it idle... and pinched the return line, fuel pressure raised to as high as I'd let it (well above 50).

I also just put in a new fuel filter and nothing changed. Problem is still there.

Compression test of 120 PSI is getting toward the low side. Most people on here report 140 to 160. Can anyone else chime in whether they have good running motors with 120 or less compression?

I'm not sure, but does it matter what elevation you're at? I'm at 5500 ft.

The vacuum levels were good and with no fluctuations.

I'm going to check the wiring harness for those rear injector locations. I know what I'm hearing is a misfire.

Just cleaned out all the injector plugs and inspected them for damage. They looked like new.

I also swapped out the ECU for a stock one and it ran the same... no fix.

What else is there? intake temp sensor, coolant temp sensor?