And the problem worsens....

Yesterday the throttle in my '90 XJ AW4 was all over the place. It would seem like if you let off the gas, then got back on it. It has been doing this intermittantly for the past couple of weeks. It started after I got yanked out of some deep mud.

Well last night it took a turn for the worse. It was so bad, it would actually jerk the Jeep while moving. Sort of boucing itself around. The RPMS would drop, then catch right back a second later, and repeat itself over again. Sometimes they would drop 250rpms, or other times it would be 1000 rpms. It will always go back to where it's supposed to be, after a brief second.

Now this morning, about 55 degrees, I go to start the Jeep and head to work. Crank crank crank with no start. On the second try, after about 7 seconds of cranking, I left the ignition in "run" to prime the fuel line. It still wouldn't start. Not having much time to bother with it, I hitched a ride from a coworker.

When I got home, it was about 75 degrees, and the Jeep started up after only a couple seconds of cranking, with no priming. WTF?

Also, lately I have been hearing sort of a "clunk" sound after the Jeep sits awhile, and I crank it over for a couple of seconds. Right after it's done cranking, the "clunk" sound happens. Sometimes it will fire, othertimes it will take a second try, but the clunk sound will not be there.

Anybody have any ideas wtf is going on?

TPS was replaced by dealership two days ago
CPS was replaced about 2 months ago
Fuel pump and filter about 3 months ago
Injectors have about 80k on them, and are from a 5.0 mustang
New starter about 5 months ago
Bought a cheapie spark plug wire tester (kind you hold over the wire), and the light blinked on all of them. I don't think the plugs are that old, but I will check them tomorrow.

I tested my fuel pressure, and with the ignition in the "run" position, it read about 36psi. With the engine idling, it read 32psi. I don't think this is normal, but I could be wrong.

Except for the wandering idle, my '01 has similar no-start symptoms. My fuel pressure is 44PSI when it should be 49.9, so maybe that's a hint. I think yours should be 40PSI, but don't take my word for it.

jldiaz said:

Except for the wandering idle, my '01 has similar no-start symptoms. My fuel pressure is 44PSI when it should be 49.9, so maybe that's a hint. I think yours should be 40PSI, but don't take my word for it.

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The thing is, it idles wonderfully. It only has these problems upon acceleration. I've noticed it nearly always happens if I am to go over some time of bump. Other times it'll be just out of no where.

My pressure gauge went up to 40psi after I turned the Jeep off. I just checked it again, and this time it read 32 pounds in the run position, and 35 pounds running. Perhaps I could have gotten the numbers mixed up the first time? But I still don't understand why my gauge went up after the car was shut off.


Could this be a ballast resistor problem?

Thanks

it sounds like you still have a crank sensor problem. is it rubbing the exhaust manifold. if it is an aftermarket sensor it could be bad. i have seen that before.

I don't think that the pressure going up is normal. Sounds more like a volume / pressure problem. If the pressure is low, the injectors will give a lean mixture.... and combustion gets slower..... and aceleration may be erratic... I also have a miss at about 2000 RPM when evening out the throttle after accelerating. My '01 doesn't use a ballast resistor; you can always check that by monitoring the voltage on the wire that goes from there to the pump. I expect to check the voltage at the pump's connector tomorrow (If my wife doesn't have any "better" ideas), but it's probably allright (It's OK at the relay).

jneary said:

it sounds like you still have a crank sensor problem. is it rubbing the exhaust manifold. if it is an aftermarket sensor it could be bad. i have seen that before.

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Yes, it was an aftermarket CPS sensor from CarQuest. It looked identical to the one the it previously had, but the mechanic who installed it said they had to modify it slightly...??

But could the CPS be causing the erratic engine/throttle problems, as well as this mornings no start?

90Pioneer said:

Yes, it was an aftermarket CPS sensor from CarQuest. It looked identical to the one the it previously had, but the mechanic who installed it said they had to modify it slightly...??

But could the CPS be causing the erratic engine/throttle problems, as well as this mornings no start?

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it shouldnt have to be modified. go get a factory one. that would also contribute to the rpm's dropping

jneary said:

it shouldnt have to be modified. go get a factory one. that would also contribute to the rpm's dropping

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Anybody else think a bad CPS may be the cause of this? The mud covered the tires when I was stuck, so I suppose it could have gotten up there. Although nothing started to happen for a day or two after.

Don't want to replace things that don't need to be replaced. Already did a TPS sensor, and I'm sure the prior one was just fine.

Thanks!!

90Pioneer said:

Yes, it was an aftermarket CPS sensor from CarQuest. It looked identical to the one the it previously had, but the mechanic who installed it said they had to modify it slightly...??

But could the CPS be causing the erratic engine/throttle problems, as well as this mornings no start?

Click to expand...

if the techinician "modified" it, make him pay for it

jneary said:

if the techinician "modified" it, make him pay for it

Click to expand...

Yes but I brought the sensor to them, and they just installed it.

The Jeep fired up just fine this morning, although I have not driven it (but it did start and idle fine).

My fuel pressure is reading at about 35 pounds at idle. I don't think this is correct.... what could be causing this? Since the engine was cold, wouldn't the computer be sending a rich mixture, rather than a lean mixture?

the fuel pressure is ok . even though it is cold it is not controlled by fuel pressure, but by volume, by increasing injector pulse.

jneary said:

the fuel pressure is ok . even though it is cold it is not controlled by fuel pressure, but by volume, by increasing injector pulse.

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at a lower pressure, less gas (volume) goes through the injector in the same period of time (pulse)....

My bet is the TPS, probably an intermittent wiper inside.

- It was just replaced (electronics usually go bad right away - infant mortality, or after a long time)

- It wont' start without it

- Cold would cause the wiper arm to contract slightly, making it open, since it seems to be marginally contacting anyway.

- bad throttle signal would make it seem like you were all over the place with the gas pedal, only worse since the throttle wasn't actually keeping up...

Check that the TPS resistance is smooth in its range with an ohmmeter. CPS would be second guess, but probably less likely. The clunk is probably not related. Can it be described as anything else besides a clunk? Perhaps a single cylinder firing poorly and only once?

On the CPS mod, I thought I read somewhere about sometimes having to elongate the mounting holes slightly to position it for a better signal. If they got too close and it hit, it's gone though and not a sometimes problem. You can check for signal during start or run if you want.

If it's cold again and won't start, and a meter is not to be found, try a hair dryer on the TPS and see if that makes it start.

-skipc

I was having these problems before I replaced the TPS. In fact, that is why I did replace the TPS.

Well the Jeep would not start this morning either. Just cranked. I popped the hood, and unplugged the CPS sensor, and plugged it back in. Jeep fired right up. So I suppose this is the culprit, or at least one of them. I am pretty angry I just replaced this something like two months ago, and now I've gotta do it again. My Jeep must think I am a money tree.

I have a theory that when the CPS does this - it isn't really a bad CPS (there's nothing in it that coudl be 'reset' by doing this) but a bad/corroded connector to it. Mine was doing that and I was unplugging to get it to work and finally used a small brush and deoxIT in it and it's fine ever since.

Your fuel pressure will rise as soon as the engine is turned off, this is normal. Cut out the CPS conector and solder wires.