Bucking erratic rpms, moving or at idle 1991 xj

so I read this whole thread, sorry for your troubles, glad you found your solution. Initially, I thought the same issues- but mine seem pretty unique. My RPMS spike and drop violently once "it begins" I couldn't ever replicate how abruptly it spikes by hitting the accelerator.

My 91,XJ,4.0 recently began- cutting out- surging back: drops to 200rpm then spikes to 1500rpm- on the road in drive at 55, in reverse, in park in neutral: once it has started it doesn't go away. If I accelerate- it will respond smoothly: So I got home by accelerating to 65mph- putting it in neutral and coasting till 50mph, (while coasting the rpms looped quickly and cyclical from 500-1800rpms).

However, it runs perfect or relatively perfect: from start up cold through operating temp- then without warning- it just starts bucking and gets progressively worse. If I let it sit 5 minutes, it will be fine upon restart. If I immediately restart-same problem.
So I can drive it to work- and it will run fine (4 miles). Just can't leave town .

I checked TPS volts which seem fine (volts in range, and no indicated spikes or drops): I am thinking coil and O2- relation to heat... I'll check Fuel Pressure.
I would not think CPS, IAC, MAP, since it runs Great until it gets warmed up and driven- strangely I drove it at my house for 10 minutes with ac on to heat it up- even trying to exceed 200 degrees- ran fine at 5mph and in park with and without AC. didn't start looping/bucking till 2 miles down the road.

Thanks in advance for any insight. I have lurked here for about 10 years- I think this might be my first post.

Re: Bucking erratic rpms, moving or at idle 1991 jx

Just to eliminate the IAC, try to catch it at an acceptable RPM (when it is cold) and unplug the IAC. I haven't tried this on a 91, liable to light up your CEL, but my be indicative of the root problem. If the surging stops, it is likely faulty input from a sensor and not something mechanical like a vacuum leak.

I'm flying blind here and just trying to throw out some ideas. Check your TPS volts and work the throttle while you are testing, works much better with an analog meter. Which direction does the TPS in a 91 work from low voltage to high voltage or from high voltage to low voltage. Should be around 5 volts *in* (some years it was 7 volts) and whatever the out voltage is throttle closed, then throttle opening etc. Don't test on the Blk with stripe ground at the TPS, test to chassis ground. Check the TPS ground to to battery negative on the low volt scale and/or do an ohm test.

Could it be a broken throttle plate return spring? I had this happen, not on an XJ, the spring broke so it was weak and not completely gone, the throttle plate was way soft, between the IAC and other sensors trying to compensate my RPM's got way wild.

How are your brakes, any whistling or sucking sounds coming from your brake booster?

Have you double checked the intake/exhaust manifold bolts to see if they are loose?

Is your purge solenoid chattering?

You are going to have to do a fuel pressure test, just to eliminate the possibility your fuel regulator is screwing up. Though the problem showing up only after the engine warms up sure seems to be a sensor issue.

Re: Bucking erratic rpms, moving or at idle 1991 jx

Sounds like a bad O2 sensor system, fuse, relay, wires or the front O2 sensor.

Re: Bucking erratic rpms, moving or at idle 1991 jx

checked fuel pressure regulator- 31psi at smooth idle, disconnected FPR=40 (stock specs/ checked out fine) when idle is Erratic and jumping- the fuel pressure does raise to 36 and down to 28 but that seems related to the idle going up and down- appropriately- like you are reving it up and down: the fuel pressure goes up and down with rpms- I assume that's normal. It stay's within normal range 28-34 when reving with accelerator and when its looping from 200-1300rpms when the problem shows itself.

I unplugged the MAP- doing so made it worse- and it generated a code.
I unplugged the Ignition Air sensor- it changed the problem- just idled High at 1300 if I recall.
I unplugged the O2- didn't seem to change much- tone was different but still looped.
I swapped the Distributer cap and rotor- no change

I have a exhaust manifold leak- usually only noticable for first 30 seconds of starting from cold- but that's been there for a few years nothing new...
No noticeable vacume leak- I checked everything over well- prior to posting had found the tps hose had a crack and swapped that out no change.
I dumped in a can of Seafoam for sh'ts and giggles thinking water or something- but no change...
Let me know if I misinterpreted any of my findings or should look at something I haven't.

Thanks

Re: Bucking erratic rpms, moving or at idle 1991 jx

Is the FPR on the fuel rail at the injectors with a vacuum line? If yes, disconnect the vacuum line and see if there is fuel there. If yes, the FPR is bad leaking fuel past the injectors into the intake with out ECU control....

Also see if the fuel pressure is steady at 39 psi with the vacuum line detached at various engine speeds, acceleration, deceleration. The fuel pressure should be steady with the vacuum line detached.

Exhaust manifold leak can also be an intake manifold leak, they share the same gasket and some of the same bolts!!!! Fix it!!!

TPS does not have a HOSE!!!

prior to posting had found the tps hose had a crack and swapped that out no change.

Re: Bucking erratic rpms, moving or at idle 1991 jx

Ecomike said:

Is the FPR on the fuel rail at the injectors with a vacuum line? If yes, disconnect the vacuum line and see if there is fuel there. If yes, the FPR is bad leaking fuel past the injectors into the intake with out ECU control....

Also see if the fuel pressure is steady at 39 psi with the vacuum line detached at various engine speeds, acceleration, deceleration. The fuel pressure should be steady with the vacuum line detached.

Exhaust manifold leak can also be an intake manifold leak, they share the same gasket and some of the same bolts!!!! Fix it!!!

TPS does not have a HOSE!!!

prior to posting had found the tps hose had a crack and swapped that out no change.

Click to expand...

I meant FPR not TPS in relation to the hose-

no fuel out FPR

I'll check if FP is steady with no vac to FPR upon decel/accel- not sure if I did that- what would that indicate?

I hate doing anything with the intake/exhaust manifold- those bolts are a challenge. guess I may have to bite the bullet- I'd hope if I did that it would be "solved" in the end... cause its such a bother. thanks for the encouragement and clues

Re: Bucking erratic rpms, moving or at idle 1991 jx

I'll check if FP is steady with no vac to FPR upon decel/accel- not sure if I did that- what would that indicate?

Click to expand...

If the fuel pressure still swings up and down the problem is in the gas tank, or the fuel filter, fuel pump, hose, or wiring to the pump.

Re: Bucking erratic rpms, moving or at idle 1991 jx

I checked Fuel Pressure again.
With FPR vac disconnected- the RPMs stayed at 40- even while reving motor- and while the motor looped on its own from 200-1300 rpms.

I am really leaning towards intake/exhaust leak. But what I don't understand is if I have an intake leak= I would think the IDLE would just be high. I don't see the connection to why it goes from 200-1300 rpms in loop (it cycles about every 4 seconds sometimes it steady's for another 5 seconds- then cycles again).

I can still drive it from cold 5 miles without an issue (runs great)- but after that I am lucky to go a few blocks: bucking bronko and wants to stall.

thanks!

Re: Bucking erratic rpms, moving or at idle 1991 jx

HUSKYnXJnWI said:

I checked Fuel Pressure again.
With FPR vac disconnected- the RPMs stayed at 40- even while reving motor- and while the motor looped on its own from 200-1300 rpms.

I am really leaning towards intake/exhaust leak. But what I don't understand is if I have an intake leak= I would think the IDLE would just be high. I don't see the connection to why it goes from 200-1300 rpms in loop (it cycles about every 4 seconds sometimes it steady's for another 5 seconds- then cycles again).

I can still drive it from cold 5 miles without an issue (runs great)- but after that I am lucky to go a few blocks: bucking bronko and wants to stall.

thanks!

Click to expand...

Then it must be a problem with a sensor in closed loop mode, like the O2 sensor. Or something overheating. Exhaust blowing on something, and overheating it, fuel injector, O2 sensor, or Temp sensor going funky when hot ? Or a combination!!!!

Focus on the O2 sensor, read the "oxygen sensor test" thread I wrote a few years ago here and run the volt-ohm meter tests on the wires and O2 sensor.

And focus on exhaust leaks near the O2 sensor.

Only other thing I can think of is a sensor plugged in backwards on the connector, TPS or O2 sensor.

If only one cylinder is sucking in exhaust from an adjacent exhaust port, caused by a leaking common gasket on the intake - exhaust manifold, it could cause the problem.

Re: Bucking erratic rpms, moving or at idle 1991 jx

I would try a new O2 sensor at this point, and make sure there is power to the O2 sensor heater!!!

Re: Bucking erratic rpms, moving or at idle 1991 jx

Thanks-I'll check out the 02. I think I am going to get an OEM Manifold gasket and replace that for starters- I was thinking the leak (maybe I even have a crack) may be adding heat to the fuel rail too- it seams hotter under the hood than I recal. Its an issue maybe not "the" issue.

I have not tested the CPS and it seams that could be an issue to consider.

Largely- I put in a rebuilt motor the within the first 2 years I owned it and although the body is "gone" on this thing its been very reliable- I have had it for 13 years and I haven't had to mess with much besides routine maintenance to keep it running. So it has all stock sensors- fuel pump- injectors. The steering/suspension/drivetrain is new to excellent condition did that last fall. I always say "it gets lighter every year" .

Re: Bucking erratic rpms, moving or at idle 1991 jx

Tested the CPS. I have resistance between B and C terminals. Supposed to have an open circuit. Probably intermittent and amplified with heat. Crossing my fingers. Obviously I have spark if this is it, its strange. But apparently the CPS is responsible for alot.

I will see if I can get one today.

Re: Bucking erratic rpms, moving or at idle 1991 jx

Ok I got it mixed up- A-B for pre 92, B-C for Post 92
Mine looks different than common pictures I see-
A lead on harness plug to CPS did test to Ground.
Assuming I have this right...From Left to right (see pic below), A (Ground), B (Voltage supply), C (Signal)
A to B- resulted in intermittent testing (couple tests were open, some were closed- I turned the motor over and tested when it ran proper and when it did not (hot) even seemed that resistance was increasing when held after shutting down)
On my 92 YJ (parked/needs frame repair) it tested good (open) between A and B- So because I can't get a CPS in this county today- the YJ will be a "parts vehicle".

Re: Bucking erratic rpms, moving or at idle 1991 jx

Well its not the CPS, apparently.

Re: Bucking erratic rpms, moving or at idle 1991 jx

Swapped out the Temp sensor-(tested bad/ but the plastic cap on it could spin round and round so it was probably not making connections) seemed to smooth it out at first but ultimately did not change the end result- seemed to drop the 1300rpm to 200 rpm to 1100 and 200. Its loop seemed not to be as fast as a cycle.
I could not figure out how to test the O2 heater/ saw the post- but have couldn't figure out which wires- Grey Grey black white on sensor. bk/wt, bk/bl, gr/bk, bk,wt on plug to harness I believe-
The OEM manifold gasket will be here tomorrow- I'll swap that when I get a chance. I hope that's it. had another look at my frame- and wonder if working on it is really smart- but I tell myself I'll fix that too- in time....................

I have to swap a starter on a Ford Focus now- wish I had to do that to the XJ instead! by looking at it you'd swear it doesn't have a starter- cause you can't see it without disassemble and a flashlight.

going to replace the manifold gasket today- I have it finally- I had them keep the first one- since it had some wrinkles due to poor handling. Couldn't believe it.... They were trying minimize the effect it could have because "its just an exhaust manifold, if it were an intake then I'd be concerned"----I said "right, well see that hole by the wrinkle- that's an intake hole- see then next hole that's and exhaust hole etc...." then I got... Oh, yea, we'll order you a new one...
Last time I bought one (last year for TJ)- I didn't like the way the guy held it- flopping around- I know its not an infant- but support it's neck!

While you are at it, make sure and check the motor mounts and bolts holding them to the motor. They can make what would normally be a small shudder into a scary experience.

Thanks- got the new manifold gasket on and everything back together. I started it up- and it definitely sounded better- (no leak). I let it idle, as I put tools away and thought it was going to be resolved. It never looped- and because it was sitting and 90' degrees out- I knew it would get hot- temps read 210 and I shut it down- thinking again- this is resolved. Then I started it back up- and it started looping nearly immediately- but smoother than before. NOT "smooth" at all- but not as drastic, shouldn't be driven like that.
So I am really uncertain as to exactly what it is. The plugs were not black- they were dry/golden.

I am going to order a new o2 sensor and a coil- as they are at the top of the list of possibles. It has to be a heat issue- or a "closed loop" issue- but I don't know about that well enough to "know". I could not figure out testing the heater circuit of the o2- didn't know what wires to test, don't completely trust my multimeter its a cheap one.

New O2 and Coil- did not resolve engine cutting out- seemed to be doun on power until then- (still runs good at cold start up till operating temp)

I don't have a clue now. Yesterday it even stuttered while accelerating, which it didn't do before. kinda thinking it could be about anything- now I am thinking the catalytic converter...

Yea problem seems to be getting worse harder to restart once it quits. But still can not drive it once it starts doing it. Seems weired that while going 55 miles per hour- the motor cuts for 1.5 seconds- and its like its hitting the brakes then jumps back to life for 1.5 seconds then cuts out- then is good for 3 seconds then back to the the start. It continues to do that till it completely dies. The motor will do that if you park it after 2 miles and let it idle too.

I don't have a clue- not alot of input here so I am guessing I have a heck of a mystery- or I don't explain anything well enough, or something.

Strongly considering giving up, biting the bullet and buying a 1999 xj that does not have "this problem". The reason I don't give up is because I think its something stupid, real stupid, that I have overlooked or possibly have no clue about. when all else fails blame the computer/ecu/pcm? Frustrated...

So to recap: new manifold gasket, new o2, new coil, swapped Temp Sensor, swapped CPS, swapped cap and rotor.
tried to test TPS with a digital meter- couldn't see any issue, could swap? but seems fine when cold?
IAC- seems fine when cold- adjusts with AC/steering/and putting in park... so should not be "it"?
MAP?
Computer?
?

least my daughters 2002 ford focus is running now with a new starter... so I can drive that... can't haul a trailer, but I can go to work...