Starts fine, drives fine, bucks, sputters and dies randomly?

Well the title pretty much explains it. 90xj 4.0L, ax-15.
Jeep starts fine, and runs. Power seems fine.

When cold, I'll roll down the block or so, then push the clutch in rapidly, and it'll try to die. Sometimes it succeeds. Starts right back up, runs fine, not usually an issue after that, as I'm careful to keep the idle up until it warms up.

Until recently, it ran just fine once it was warmed up.
Yesterday, it tried to die on me TWICE on a 30 mile drive. The first time was at highway speed. Doing about 70mph... 2700rpm or so. just kind of tried to give out. rpm's dropped sharply, I gave it more gas, had to sorta pump the throttle, and it caught back up. I made a quick stop at the pet store then went the rest of the way. Maybe two miles from the pet store, sitting at a stop light, it started bucking, sputtering, popping, like the distributor was 180* out of time. It was bad. I tried throttling it, it didn't do a thing. I ended up killing the engine. I restarted it, it idled at about 3K, but the light turned green, so I just rolled with it. Rpm's dropped back to normal-ish, and I finished the drive home.

Today, on my way to go 4-wheeling, it decided to straight up DIE in the McDonalds drive-thru. Engine was warm, and basically idling. I'd been goofing around lightly revving it, but nothing unusual, then just dead.

I'm kinda at a loss as to what it could be. It's not acting like a bad CPS, 'cause CPS would tend to just die and then not re-start for a while, am I wrong? I'm sort of thinking fuel pump or perhaps TPS?

Grounds, battery leads, charging cable wires, and so on have all been replaced about a week and a half ago. O2 is pretty new (but POSSIBLY bad), just cleaned the CPS, new MAP sensor...

Any help with this would be greatly appreciated, I'm pretty confused with it.

ASD (auto shut down relay)? Fuel pump.. IF you can, put a mech. gauge on the fuel rail and post your results.

i was thinking fuel pump too. i had the same issue. jeep would stall after a couple of miles.

Could also swap relays underhood and see if that changes anything. Could be simple as the relay failing.

Which relay is for the Fuel pump? I'll gladly throw a relay at it.
I'll get my hands on a fuel pressure gauge, and post the numbers.

The order from front to back of the relay center is:
Oxygen Heater
Fuel Pump
Power Latch or B+
A/C Relay

CEL?

Frank Z said:

CEL?

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Not in 90 with Renix.

d'Oh!!
Misread that...thought I read 98.

Sorry....

tayman said:

I'm kinda at a loss as to what it could be. It's not acting like a bad CPS, 'cause CPS would tend to just die and then not re-start for a while, am I wrong? I'm sort of thinking fuel pump or perhaps TPS?

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Usually CPS's tend to flat out die, or more commonly, they knock off when the engine gets warmed up and then come back to life when the engine cools down. With that said however, there have been a few cases where people reported intermmittent power loss shortly before it died all together, so I wouldn't rule out a bum CPS just yet. Search around on here for how to test them (not sure if testing them on a renix is different from a HO/OBD 4.0).

o-2 sensor , had identical problem on a 98 , after throwing numerous at it including a 225 cdollar fuel pump , and other parts turned out to be the o-2 , get it hooked up to a puter and eliminate the guess work. just trying to save you some b.s. that i went through . live and learn thing , yep , im slow

Never heard of an o2 sensor completely shutting down an engine. You can unplug it and run it all day long if you want...although it'll run way rich by default, it still runs.

UNCC_99XJ said:

Never heard of an o2 sensor completely shutting down an engine. You can unplug it and run it all day long if you want...although it'll run way rich by default, it still runs.

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Yup, what :lecture: he said. I've broken my O2 a couple times, and ran with a gaping hole in my exhaust where the sensor used to be. It's not the O2.

As for Fuel pressure numbers:

Key in the "on" position, it sits right about 37psi

Turn the engine on,

Idle pressure is something close to 27-29psi

Rpm's do nothing to affect pressure.

Spec is more like 49 psi at idle, right?

UNCC_99XJ said:

Usually CPS's tend to flat out die, or more commonly, they knock off when the engine gets warmed up and then come back to life when the engine cools down. With that said however, there have been a few cases where people reported intermmittent power loss shortly before it died all together, so I wouldn't rule out a bum CPS just yet. Search around on here for how to test them (not sure if testing them on a renix is different from a HO/OBD 4.0).

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Yeah, I've had CPS failure before. It was the first gremlin I had to deal with when I got my jeep. It was my first thought on this issue, until I realized that heat had nothing to do with it. I'm sticking with the "not the cps" thought for now... it's still pretty new. (well, a year or so anyway)

tayman said:

Yeah, I've had CPS failure before. It was the first gremlin I had to deal with when I got my jeep. It was my first thought on this issue, until I realized that heat had nothing to do with it. I'm sticking with the "not the cps" thought for now... it's still pretty new. (well, a year or so anyway)

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They're fun to deal with aren't they?

Given your fuel pressure readings I would concur that it's not a CPS. Another thing I just had an "oh yeah!" moment on...last year I worked on a buddy's 88 Renix that would sputter, be hard to start, and eventually died on the trail one day. We also got low fuel pressure readings once we got it home and checked it. We bought a new pump for it and dropped the tank only to find a brand new Bosch pump already in the tank, and to find one of the hoses had popped off. Popped the hose back on and it's been running great ever since.

UNCC_99XJ said:

They're fun to deal with aren't they?

Given your fuel pressure readings I would concur that it's not a CPS. Another thing I just had an "oh yeah!" moment on...last year I worked on a buddy's 88 Renix that would sputter, be hard to start, and eventually died on the trail one day. We also got low fuel pressure readings once we got it home and checked it. We bought a new pump for it and dropped the tank only to find a brand new Bosch pump already in the tank, and to find one of the hoses had popped off. Popped the hose back on and it's been running great ever since.

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HUH! well that's kinda handy. Annoying as hell, but a nice cheap fix, nonetheless.

Can I get confirmation on the pressure spec? Looking around I've found 49psi, but I think those were newer, HO engines...

tayman said:

Yup, what :lecture: he said. I've broken my O2 a couple times, and ran with a gaping hole in my exhaust where the sensor used to be. It's not the O2.

As for Fuel pressure numbers:

Key in the "on" position, it sits right about 37psi

Turn the engine on,

Idle pressure is something close to 27-29psi

Rpm's do nothing to affect pressure.

Spec is more like 49 psi at idle, right?

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Spec on fuel pressure for renix is 29-39 psi. Your fuel pump sounds like it is fine.

It should run from about 29 to 35 psi with the vacuum hose attached to the fuel pressure regulator, which is located at the forward end of the fuel rail on the Renix years. With vaccum hose detached it should read about 39 psi, I have seen 27-41 psi peak to peak on good Renix fuel systems.

The fuel pressure should jump up when you rev the engine, then drop on deceleration!!!!!! (with the vacuum hose attached to the FP regulator).

HO engine fuel pressure is 49 psi.

Hard to say what your problem is, but it is time to check the hot and cold resistance of the CTS and IAT (also known as MAT, or.... intake air temp sensor).

My money would be on the TPS being on its last legs. Your symptoms are typical of a bad TPS. The internal contacts on the TPS wear with use, and get worn, noisy spots that confuse the ECU, causing erratic fuel injection rates.

I had a brand new Renix Auto Zone CPS 3-5 months ago, that drove me nuts with random no starts, but was fine once it started. It had a cold junction wire connection inside the CPS that finally failed after 8 weeks of hair pulling, thus confirming the gremlins location. I replaced it with a CPS from Rockauto.

Did you do the CPS boost signal procedure, moving the Renix CPS closer to the flywheel? Cranking DVM meter, AC voltage on the isolated CPS needs to be about 0.50 volts, or it will give no starts periodically.

I had cold engine no starts and random high idles once, hot when my IAT went bad.

So far, the TPS sounds like the top suspect. Check the test procedures in the Lunghd... link below for the TPS. It also has test data specs for the CTS and MAT.

How did you "Clean" the CPS?????

Ecomike said:

Spec on fuel pressure for renix is 29-39 psi. Your fuel pump sounds like it is fine.

It should run from about 29 to 35 psi with the vacuum hose attached to the fuel pressure regulator, which is located at the forward end of the fuel rail on the Renix years. With vaccum hose detached it should read about 39 psi, I have seen 27-41 psi peak to peak on good Renix fuel systems.

The fuel pressure should jump up when you rev the engine, then drop on deceleration!!!!!! (with the vacuum hose attached to the FP regulator).

HO engine fuel pressure is 49 psi.

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The fuel pressure doesn't move at all with revvs. It just stays about the same. Bad regulator? clogged line? I'll try pulling the vac line off the regulator and see if it makes a difference.

CPS went bad in just months? that's quick... I also have occasional no-starts... but starting fluid takes care of that.

Cleaning the CPS consisted of removing it from the bellhousing and wiping the grime off of it.

I have not done a TPS check... I may just replace it, seeing how it's 21 years old 'n all.