Banging head against wall!


#1

Related to my jeep injector & no rev problem . This jeep has an excellent body & interior & high mileage . I bought it cheap knowing it needed an engine . When I bought it I drove it about 40 miles home & the engine ran like crap , the ses light was flashing part of the way . I knew the compression was low . Taking into consideration the mileage & the crappy running engine , according to what I’ve read I wouldn’t be at all surprised if it had cat problems . I’m now going back to the no rev problem .
As I’ve stated before , with the alternator wires disconnected it seems to rev up fine . With those wires connected it won’t . Obviously the cats can’t be completely clogged . Does anyone think one or both of them could be partially clogged causing the pcm to limit the rev’s & the alternator wiring could have anything to do with this ? Sounds like a reach to me but who knows whats connected to what in these computer controlled engines . Also , wouldn’t I get cat related codes if there is a cat problem ?
I could have dropped the exhaust system from the exhaust manifolds but no cat related codes , no 02 sensor codes & I’ve been concentrating on the injector problem . For anyone that hasn’t followed my threads , I installed a low mileage engine that ran fine in the jeep I removed it from .


#2

only possibility I can think of (with my very limited experience) to explain the alternator connected/disconnected issue is that there is a short on the alternator output, or it has an internal short, and the engine doesn’t have enough power to drive it against the short. But I would think that several hundred HP could do that, even if it has to break or slip the belt.

Question, without the alternator it seems OK. But that is just the engine with no load. Will it drive under that condition? Can you move it 10 feet? That is, can it supply meaningful power?


#3

I also tried the alternator off the original engine , same problem . Alternator output measured on battery is 14.3 volts . Didn’t have alternator/no rev problem in doner vehicle . Don’t think alternator itself is the problem .
Jeep isn’t licensed or insured yet so can’t road test but it moves around ok in my driveway . It will pull up on ramps , no problem . Just when the rpm’s hit about 2200 it acts like it hits a limit switch or governor with the alternator wires connected . It doesn’t do that with the alternator wires disconnected .


#4

This must be the fifth thread on this misfire/rev limiter problem. This is a designed feature within the PCM. When a performance related fault is detected the PCM will lower the rev limit to 2000 RPMs in neutral. By disconnecting the alternator you were able to defeat the rev limiter but you will need to get the engine running on all six cylinders eventually.


#5

So moving around your driveway, with alternator disconnected, you can get more than 2200 RPM?

So if the problem is not internal to the alternator, it could be a short in it’s output. Do you have a clamp-on DC ammeter you can connect? it would need to be a 100 amp range. If not, a voltmeter on the alternator’s output may tell you something.

Or, if this car uses the engine computer as a voltage regulator, the problem could be there. Or if it has a separate regulator, did you check that?


#6

I had all 6 cylinders firing & engine was running really smooth , still wouldn’t rev . Have a new pcm on the way . Still don’t understand what the alternator would have to do with a rev limiter that was activated because of a cylinder misfire . Obviously the rev limiter was activated when all 6 cylinders were firing .


#7

Depending on how old this vehicle is, the alternator could be under the direct control of the PCM. It could (falsely) detect too high an output from the alternator and limit the RPMs for that reason (Just a guess, who knows what goes through the minds of programmers).


#8

If you still have the old engine, take every sensor and everything that has a wire from the car harness attached to it and swap it onto the new engine. If it still won’t rev, then the issue has to be mechanical, but I suspect that it is a sensor issue.

If your vehicle or the vehicle the new engine came from has a BCM, that could also be an issue, but swapping every sensor and every item with wire(s) attached should solve the issue. Start with the distributor and then the crankshaft and camshaft position sensors.


#9

Very possibly Nevada is correct . Just did some more googling & several people are saying it does have a rev limiter in park & neutral , most people are saying it’s about 3900 rpm’s though . I also suppose the alternator wiring could be integrated somehow .


#10

When you connected cylinder #4 injector to cylinder #5 wiring you still had an active fault for injector #4 open circuit, correct? Injector, coil and misfire faults will reduce the rev limiter to 2000 RPMs in neutral.

Did you try using your other PCM with matching immobilizer module?


#11

No the P0204 went away but as I said it was running smooth & I pulled the coil wires to # 4 & # 5 just to satisfy myself that both cylinders were indeed firing & they were . I then cleared the codes I created by pulling those wires & restarted engine & then it ran rough with a code for # 5 coil & found no ground signal for # 5 coil . What installing a jumper wire from # 5 injector to # 4 injector had to do with # 5 coil ground I have no idea .
The misfire now on # 5 coil could be causing the rev limiter to limit revs . I’ve quit until the new pcm arrives .
As far as trying the other pcm & immobilizer goes , You had mentioned trying that before & said I might try swapping the ignition switch , immobilizer & pcm . The key is stuck in the switch in the other jeep & won’t come out & a lot of reading I did said the pcm’s had to be programmed to the vin of the vehicle so for those two reasons I didn’t swap everything . Might have worked though . I did just stick the other pcm in & the engine would start & run for a couple seconds & shut down .

The main reason for starting this thread was after I install the new pcm , where do I look if I still have the no rev problem ? The new pcm is plug & play & hopefully all my problems will be gone after installing it .


#12

w/respect to your clogged cat suspicion, have you measured the intake manifold vacuum? I think there’s test shops do where they rev the engine to a certain rpm (2000 I think, with the car parked) and the intake manifold pressure should be 2 or less, more than that they suspect a clogged cat.

If you know somebody with a truck and the same engine, you could install the suspect alternator on it and see if it does the same thing. But I think Nevada above has identified the source of the rev limit, and somehow by disconnecting the alternator, that defeats it. Maybe they designed it that way so the techs could easily defeat rev limits to conduct diagnostic tests in the shop.


#13

I just recently bought a vacuum gauge set but no I haven’t checked vacuum . I’m not doing anything else to it until the pcm I ordered arrives & I install it & see what happens . I also think there’s a very good chance Nevada has it right . I tried another alternator on it & it made no difference .


#14

@Sloepoke

Aren’t you going to have to set up and marry the new pcm to the vehicle . . . ?!

That has pretty much been the rule of thumb, for a long time now


#15

The place I ordered it from wanted the vin # , mileage & # off my current pcm . They said with that information they could program it to my vehicle & I could install it & it would be plug & play .