Bucking above 2000 RPMs

I have an 88 MJ with a 5 speed. I was having intermittent high idle problems for a long time, about 2200 RPMs and the only way that I could fix it was to unplug the IAC. I have replaced the IAC with a new unit but it made the idle problem worse (possible bad new unit). I am also having a bucking issue that seems to happen above 2000 RPMs, especially when maintaining speed or lightly accelerating. The bucking happens regardless of how the truck idles. I know that I should fix the ICA issue but that shouldn’t be causing the bucking. Not having an IAC should really only affect the truck when cold at start up, correct?


I have done the following:

Replaced and adjusted the TPS
Replaced the CPS
Cleaned and added grounds
Cleaned the C101 connector
Replaced plugs, wires, cap and rotor
Replaced the fuel filter and checked fuel pressure


The only things left that I think could possibly be causing the bucking problem would be a bad O2 sensor or a faulty EGR.

is it just bucking or does it also backfire?

yes, i would check the egr and yes the iac should have no effect when driving.

i wonder if a stator/cam sensor could be the issue? this would not allow the correct fuel mixture at higher rpms.

I've bypassed the EGR and I'm still getting problems. It is starting to get worse and actually backfire through the exhaust.

Could a bad O2, air temp sensor, or bad/leaking injector cause this?

Did you ever pull your distributor? If so, it sounds like a distributor indexing problem.

No, I haven't pulled the distributor yet. If it were an indexing problem wouldn't the miss happen throughout the RPM range?

Also, under full throttle I don't get any missing or backfiring.

My MJ did the same thing only on steady cruise or light acceleration, at heavy load, it smoothed out. I changed the Manifold air tem sensor and things got better.....

I had some TPS ground issues, but most of my 2200 RPM buck went away with a new O2 sensor. At the same time as I replaced the O2 sensor, I repaired the O2 sensor wire, behind the power steering pump where it had been cooking on the manifold.
Something else I noticed, my motor wouldn't run to redline either, when it was bucking. After the new O2 sensor, it would run well above redline.
I also smoothed my idle out, by ohm testing and replacing a couple of very high resistance spark plug cables. At the time my Renix had multiple issues.
If it is idling above 2200 and it's not the IAC, it may be a major vacuum leak. My Renix seems to adjust to minor vacuum leaks and the idle doesn't change much, though it does have a tendency to hunt (idles seems to cycle up and down around 200 RPM as the ECU tries to correct the air/fuel mix).
Mike posted, an on the vehicle, O2 sensor test, I think it was in the Renix Files thread.
Intermittent high idle for me has been either a wet TPS or loose manifold bolts. I had one hunting at idle issue +/- 400 RPM, that turned out to be a TPS ground issue at a crimp under the tape on the firewall. The bolts seem to back out of my intake manifold every couple of years, a recurring problem (be really careful with the studs at the ends of the manifold, I usually torque these very carefully).
I've had most of my EGR issues at or near idle, stalling etc.
Crossfire has been an issue in the past, but is usually random, the cap gets a coating of carbon as it wears and can occasionally cross fire. You can normally see a lightening shaped scorch mark inside the cap between electrodes. My distributor caps seem to wear out faster than my plugs generally (maybe the cheap caps I get here).

x2 on the O2 sensor (at least check it), you can bench test them with a propane torch and DVOM, google it or search 02 sensor test on youtube.

Carone said:

No, I haven't pulled the distributor yet. If it were an indexing problem wouldn't the miss happen throughout the RPM range?

Also, under full throttle I don't get any missing or backfiring.

Click to expand...

Normally it will start as a given rpm and get worse as the rpms increase.