97grand4.0

On mine the 97 it's under the rotor, they call it the switchplate. IDK about the renix at all but have heard you do pull the dizzy. If the shaft is loose with mine it can be bad enough that it physically hits the cks, and of course if is loose it will distribute the spark quite unevenly.
Pretty easy to check injector resistances, unplug each one and test it with a meter. You're looking for one with way out specs.

eb1

Thx for the cam sensor info; I didn't know if it played a major role or not for the ECU.

Yeah, doing the coil before doing the wires seems out of order... I did buy the wires before I bought the coil. While I was waiting for the new wires to arrive (and before I bought the coil) I also tried swapping-in a set of very low mileage spare wires I had from another rig and couldn't detect any performance difference so focus shifted towards the coil.

eb1

Thx for the how-to info; I'll report back on the resistance after I check it.

eb1

I installed the new plug wires then fired up the XJ; it was running extremely rough, way worse than last weekend, with severe stumbling and hesitation, like when you pull the MAP vac line; I thought it was going to stall instead of idle. What the %#$@! is going on?

Went under the hood... Surprisingly, I could hear a definite air-sucking noise (that has not been obvious before) so I started checking vacuum lines. The noise seemed to be coming from the air box so I gave it a good re-seating motion; noise stopped! And the engine came to life. And I could not get it to hesitate or stumble at any rpm. I took it for a test cruise with the same results -- no hesitation/stumbling at any rpm and/or under load. Finally, some sort of breakthrough to indicate air vs spark vs fuel as the main issue. I'm shifting my troubleshooting focus to the vacuum lines/harnesses.

This engine performance issue has been an intermittent problem so I know the improvement is temporary, that the problem will return when the vehicle is driven and the air box and/or vacuum lines bounce around and change position. At least I can now focus on vacuum and I won't be guessing about the fuel supply, the sensors, the ICM, etc. I picked-up the new/replacement vacuum harnesses just to be prepared.

Many thanks for all the advice and suggestions. This engine hesitation/stumbling issue is getting much closer to being solved.

eb1

Started the XJ for the first time this morning a couple hours ago; severe stumbling/hesitation, might stall, just like yesterday morning. Played with vac lines and couldn't get it to respond. Started worrying that maybe I had a false positive when vacuum appeared to fix it yesterday. After much fiddling it finally smoothed-out but I couldn't say for sure if moving around the vac lines had been the fix.

So I swapped in the new vacuum harnesses. The XJ started right up and smoothed out right away, like it should; test cruise verified that the major stumbling/hesitation is gone. I'm thinking the vacuum harnesses solved my issue but I'll give it some time and miles before I get too cocky.

DFlintstone

A vacuum leak usually shows up at idle while vacuum is highest. Yes yours does seem low. Did you check the throttle body to manifold bolts/gasket? Thinking when you giggled the airbox maybe something there shifted.

You mention the idle fluctuation. Mine use to, but I learned AFTER changing the 02 sensor, that that is was what caused it. Only other issue was some mileage and carbon at the tale-pipe.

I made this thread to test meters for 02 testing. It was misled by a moron, and not supported by members that could have been of help. Came out a mess, but still there is stuff.... Turns out it seems a cheap analogue meter is right enough to check a renix 02 sensor. After I found mine to be stone dead and changed it, my idle stopped cycling. (also the needle then fluctuated around 2.5V as it should)

Right on top is Ecomikes informative link, then a bunch of mess follows..> https://www.cherokeeforum.com/f2/ren...ce-rms-133153/

eb1

Checked injector resistances this morning:
* injectors 1,2,3,4,5: all measured 16.3 Ohms
* injector 6: did not measure; clip was hard to remove.
Glad I tested, they seem to be okay.

Then I hooked up the fuel pressure gauge just to check that fp is still low:
Key-on = 32psi
Idle = 29psi, 36psi with vac line pulled from fuel pressure regulator
Yep, still 2-3psi low.

eb1

Thanks, DF, for the O2 info/link; "cycling" is a good word for describing the changing idle levels; I may get to testing the O2 this evening -- will try the connector backprobe method if/when I do it.

But here's today's performance update: stumbling/hesitation at first and then it straightened out. School of Hard Knocks is back in session.

When I started the XJ today the idle was fine but when I gave it throttle there was hesitation/stumbling -- I did not expect any hesitation/stumbling; thought the vacuum harnesses I installed yesterday had solved that issue.

So I got behind the wheel and played with the throttle with tranny in Park. Moderate hesitation/stumbling above 1200rpm; at 2000rpm it's like it hits a wall, major missing, the tach dives; continued to try varying levels of throttle for a minute or two with no improvement.

Then I remembered the wife telling me that when she was driving two weeks ago and had the hesitation/stumbling occur she was about to pull over when she put the tranny in Neutral and revved the engine a bit; the engine stumbling went away, she was able to shift back to Drive and continue driving. Sure, whatever, I'll try it. So I shifted to N, revved a couple of times and then gently applied the throttle -- 1200, 1500, 1800, 2000rpm and no hesitation/stumbling. WTF? Shifted back to Park and gently applied the throttle -- across the rpm range there was no hesitation/stumbling! WTF? Next, I took the XJ for a test cruise -- it drove fine with no hesitation/stumbling.

Yesterday, playing with and then replacing vacuum lines appeared to cure the hesitation/stumbling. Today, it seemed that shifting from P to N to P cured the hesitation/stumbling (which really makes no sense to me). Maybe it's just a matter of having the engine run for a while that cured the hesitation/stumbling symptoms on both days?

ECU, ICM, and O2 have not been replaced yet; I'll check the O2 next.

cruiser54

Perhaps a set of 746 injectors from Programbo here on CF is in order......

eb1

Thx for the suggestion, cruiser. I put in the last set of injectors in 2002, about 70k miles ago, to replace leaking originals; used stock replacements from Jeep parts department.

Makes sense that new injectors could improve/restore the fuel pressure. I'll read about the 746s.

DFlintstone

The system is always correcting to get the fuel mix, (injector pulse width) where it belongs. It takes a rough shot with the vacuum to the MAP, then with the 02 sensor it fine tunes it. My guess is with a good 02 sensor, a few Lbs. of fuel pressure should be correctable. Note that if the ECU freaks and doesn't like the sensor inputs it will switch to default settings, mainly just using the MAP for fuel mix. ..Wonder if there is a fine line with low pressure and a failing 02 sensor where intermittently it goofs up? An 'analogue" meter has a needle you know. Normal digital's have a "sample rate" that may well just show gibberish with a fast changing voltage. (a cheap analogue meter defeats this with shear inertia, the needle itself wavers). A $300 fluke digital doesn't have that "sample rate" problem, is I guess what I've gathered. They sample way faster, so the reading can "sweep".

Btw, "clearing it's throat" revving it is something done when carbon had built up. Might not hurt to pull a plug when it's messing up and see if it's white or black. Something with the CAT? Back pressure? See Cruisers vacuum test. You do have low vacuum...why?

Anyway pulling a plug, and the vacuum test are things that can be done in a few minutes.....

The tack diving is bothering me. I know the CPS was changed, still opening the hood, unplugging the CPS plug, attaching a meter set on an AC volts scale that includes 1-5, (usually the 20V scale), Directly to the two wires to the unplugged CPS, and cranking it only takes three minutes. If your reading is below 3.5 ACV, the CPS not signaling right, might be why the tack drops out. See cruisers CPS test.

eb1

Important update: wanted to debunk the voodoo about switching gears somehow impacting hesitation/stumbling...

Let the XJ sit all day then started it an hour ago; it idled fine but as soon as I gave it throttle it had moderate to severe stumbling/hesitation; it would go back to idle with no drama when I let off the throttle. So I did nothing; didn't go under the hood; didn't shift the tranny; just let the XJ idle in Park. After about three minutes, the problem healed itself; when I revved the engine there was no hesitation/stumbling and I could not get the rough running to return. Shut it down.

That must be what happened this morning when it was running rough -- at about the same time (2-3 minutes after starting) the engine was going to smooth out by itself, I shifted the tranny from P to N to P; when the engine smoothed out it seemed shifting the tranny had made the difference but it was most likely not connected at all.

More test data... Waited 15-minutes and refired the engine; idled fine; no hesitation/stumbling when I gave it gas. Waited 50-minutes and just went out and refired the engine; idled fine; no hesitation/stumbling.

So, whatever is causing the hesitation/stumbling under acceleration, it now seems to be much better than it was:
* only occurring at initial startup after engine has sat for hours?
* goes away on its own after idling for a few minutes?
* does not return on warm starts?

I'll check it again tomorrow morning.

DFlintstone

Quick reply....(I'm gofer hunting in the garden, .222 does work). Please look again at my and Ecomikes stuff on the 02 heater circuit. It has a plane old dumb 12v resistor heater to make it kick in faster, and it's own relay in that bank of four. (might be 02 heater relay only gets power from the fuel pump relay while it's running. It might just keeping getting hotter, and hotter, with no flow past it, so only comes on when the fuel pump has a signal from the ECU that it's running. (our fuel pump relay fills functions that the ASD relay does in OBDI&II. I mean check the unplugged Jeep side of the connector for 12 ish with it running...Btw middle is ground, one end is the 5v feed from the puter, the other end of the three is the 12V from the 02 heater relay.

eb1

I appreciate the help, DF.
I did not get to testing the O2 today so it will have to wait until next weekend. My DMM is good/fast and I'll try it on the O2 test to see if it can keep up with the changing signal.

Plugs are clean, just changed recently; there was some carbon buildup on the previous plugs but just on the top of the threads, not on the electrode or tower; will check them again at some point.

I just read cruiser's vacuum test; thanks for the idea to check for exhaust restriction.

The tach diving is a mystery; I thought CPS, too, which is why I bought a new CPS a month ago even though the old one tested fine. New one from Napa tests fine, too, on both resistance and cranking tests; the new CPS got .48v on the cranking test.

eb1

Will do; just found the quick test from the Service Manual for the O2 Sensor Heating Element -- just check resistance across terminals A and B of the sensor connector; should be between 5 and 7 Ohms; replace sensor if get infinite resistance. I'll try that this week.