Odd engine problems

I have a 93 with a 4.0 and an automatic and have a couple strange issues.

Every now and then after startup it won’t rev past 1000 rpm. It hits 1000 and drops very quickly back down to idle (around 500) then revs back up quickly. It’s like hitting hard rev limiter, happens very quickly load or no load for a short time (30-60 secs or so). If you floor it it will go away quicker than if you let it idle.

Next issue is a lack of power until you find the perfect accelerator pedal spot. You have to hunt for it but it’s there about mid throttle (seems like same spot every time). It’ll go from Suzuki samurai power to real 4.0 power once you find that spot. It’s not a down shift or a transmission slip. No sounds when it happens.

Won’t rev past 4000 99% of the time. It wouldn’t EVER go past 3800 rpm until I changed the FPR.


A good amount of the time it takes a couple try’s to get it to start from cold.

It has a misfire every now and then. Maybe once every couple mins.


I’ve changed:
CPS (it was bad and caused other issues)

TPS (no change, but it was a crappy orielly brand and did ohm out well but didn’t try to see how smooth it changes)

FPR (after changing it went past 3800rpm to 4000 with it going to 5000 once)

Cap, rotor, wires and plugs

Fuel pump (original went bad. The new one was once again orielly, Napa is only other store on island and they close earlier).

Cleaned IAC

I also ordered rebuilt injectors today.

I think that’s all I’ve changed. I’ve been troubleshooting for awhile. I didn’t know how horrible orielly was until I changed 4 new vw starters, 2 new fuel pumps in three different cars (including the current Jeep) I got from them.

My thought about the throttle issue is the TPS but I don’t know how much influence it has on the fuel map.

I think the rpm limit is due to a lack of fuel... it will pop when it hits 4000 and when it does the 1000 rpm thing. Figure injectors are old..

I’m planning on replacing the o2 sensor next. Figure it’s old and couldn’t hurt.



Trouble is I live on a remote island in Alaska and have to have just about everything shipped to me. I want to make sure I’m going in the right direction so I’m not waiting a month because I missed something silly but I don’t have a disposable income to just order everything under the sun.

Oh and I have the attachment to hook up my gauge to check the fuel pressure. Looks like about a week out though.. it has a lot of pressure when running, good pressure right after shut down but seems pretty low a min or so after shut down. I figure it could be injectors (not leaking out of sides) or valve in fuel pump..

I also changed fuel filter and one time but will do another tomorrow.

When I changed my IAC from OEM to aftermarket I have this issue now.
Engine revs to 2k on startup. My OEM IAC caused the car to almost die and I would have to hold the Gas to keep her alive for the first 3 mins.

I would recommend junkyard or oem IAC

It's either IACV or you have one HUGE vacuum leak around the exhaust manafold.

Why the IAC?


I’ve checked for vacuum leaks.


Echlin brand is only thing available here. I can order online if needed but “junkyard” here doesn’t have Jeeps. It’s not a real junkyard lol.

IACV and Crank position sensor are the only things that come to mind if u have checked for vacuum leaks and have no CEL

if u do the O2 buy a NTK

Just for the heck of it use the key method and check for stored codes. Write down the codes, disconnect the battery and touch the positive and negative battery cables together. Let it sit for an hour (cool down) then start and drive, works better if you drive most types of roads from stop and go to cruise. Recheck codes.
There is a whole protocal for ECU relearning, I found just driving it through most all types of driving works just as well, maybe better.

Have you volt tested your battery, motor running and the battery after sitting all night? Low voltage can cause all sorts of issues.

You said you checked for vacuum leaks, but have a second look at the one to the MAP sensor. Inspect the wiring harness that runs along the injectors and where it crosses over to the firewall. I've been seeing a rash of problems there lately, which can be exacerbated by bad motor mounts causing more flexing. I would ohm out the coolant temp sensor since you've got a meter.

I'd be tempted to unbolt the exhaust at the downpipe and see if it revs better, to eliminate a clogged cat.

For the first few minutes after starting a cold motor the ECU doesn't use the O2 sensor. Then the O2 sensor sets fuel after the initial warm up period. If it screws up right away after startup in the first couple of minutes, there is less chance it is the O2. If if it screws up after a few minutes of driving it can be the O2. The O2 often doesn't set a CEL/code until it is complete junk, before it completely fails it may screw up some but not set a code.
It would probably be a good idea to check all of the O2 wiring front and rear. Paying special attntion where the wiring comes up the front of the motor, especially if you have changed a water pump or done some power steering work. The cable in the wrong path can burn on the exhaust manifold and is really hard to see.

Just some ideas. Other things can go wrong other than sensors, in fact when I'm troubleshooting I check the wiring and other basics before I check the sensors. Checking the peripherals first can save a lot of money on sensors.

lawsoncl said:

You said you checked for vacuum leaks, but have a second look at the one to the MAP sensor. Inspect the wiring harness that runs along the injectors and where it crosses over to the firewall. I've been seeing a rash of problems there lately, which can be exacerbated by bad motor mounts causing more flexing. I would ohm out the coolant temp sensor since you've got a meter.

I'd be tempted to unbolt the exhaust at the downpipe and see if it revs better, to eliminate a clogged cat.

Click to expand...

Good advice

Awesome responses! I’m at work now but will do the key method for codes.

When I did tps I reset the computer and no change.

I’ve been tracing wiring looking for bad connectors and burned wires and will go over it again. I just double checked for vac leaks and none so far.


No cat on it to plug and I put a new muffler on. Didn’t change anything when I did. I’ll make sure there isn’t any squished spots I might have missed.

I’m pretty new to EFI but understand enough to get into trouble (almost went megasquirt on a vw) but I’ll look around for the map sensor.


Thanks for the responses. I’ll update when I check everything mentioned above. Might be able to on lunch break.