First off, jeep is 87 yj. I have swapped in a 5.2 magnum from a 93 cherokee, 5 speed from a dakota. Pcm from 94 dodge van.
Having issues about half throttle and over. It goes lean and runs out of power. Throws no codes. All sensors seem to test ok. New cps, new o2, new fuel pump.

Stumped.

 

There isn't much to go on here, so first, I'll assume that the engine is tight and that you have all the timing set up correctly. Also, you didn't say what the ECU was controlling before it became part of your jeep configuration, if it wasn't a 5.2, that could be your problem - you need a match with the engine, sensors, and ecu. I'm using the term Engine Control Unit here because that's about all the OBD1 engine computers did. I believe that there was also another control unit for the transmission and that could also be your issue if it's not matched or connected properly - it could be shifting at the wrong times giving you the lack-of-power symptoms.

 

Original pcm was the original pcm from the 93 grand cherokee that motor and harness were from. Original pcm couldn't be used due to security system. Fairly sure that the 93 jeep and 94 dodge used same sensors.

No trans computer on either vehicles and mine is a 5 speed so its not shifting issues.

Motor is tight, distributor is indexed correctly. I assume cam timing(timing chain) is good as it shows none of the typical chain issue problems.

 

I can't quite follow your explanation on "original pcm", so I'll just assume that you mean that the 5.2 has the ECU it had originally along with all the associated harnesses.

I'll take this to mean that you have a manual transmission - there are also 5 spd automatics as well.

OK, what tests have you done on the various systems on your engine:

• Fuel Pressure - idle & load to check regulator operation?
• Vacuum (idle, increased rpm, load)?
• Spark condition at increased rpm?

Some other questions:

• How are you checking for diagnostic codes?
• Are you familiar with vacuum gauge diagnostics?
• Does the problem occur on open loop, closed loop, or both? Here are some guesses. If it's different between cold and warm operations, you could have a sensor problem. If it operates the same cold and warm, you could have vacuum or regulator problems.

 

Fuel pressure has been checked at idle was 31 with regulator plugged in and 39 with it unplugged. Have not yet done a load test, waiting on longer hose.

Vacuum at idle is 17", raises over 25 with some rpm.

Spark condition is unknown. Have no way of testing.

Issue is presesnt cold or warm engine. Its only when accellerating. There is a spot in the throttle when pushing more the motor lays flat and just doesnt produce power. No backfiring or bucking it just is flat. If you let off throttle to a point it will start to pull good again.

Diagnostic codes are just from check engine light. I get code 12, 55 and a couple trans codes(pcm is from automatic and i run a manual.)

 

Fuel pressure driving is 31-41 depending on engine vacuum(vac regulator) 41 steady when running issues are present.

Timing is 10 btdc at idle. At about 30 at 2500rpm.

 

Load and unplugged should read about the same. It's the decrease in vacuum that you get under load or when you stomp on the accelerator that causes the regulator to allow more fuel.

Is the vacuum steady or is there fluctuation showing on the gauge?

At this point, this probably isn't the cause, but check with some of the local parts suppliers if they can test your coil. It would be good to confirm that it's OK.

Off hand, I'd say that the sensors and ECU are operating as they should & the problem is a physical setting - a quick guess would be timing.

That sounds fairly normal for your setup.

That sounds about right. When your problem is present, the engine is under load, so your vacuum will be low causing the regulator to increase fuel pressure.

If you've set the distributor by a timing light or scope then it's probably set wrong. On these engines, there should be a lock-down position for the distributor and the ECU does the spark advance as necessary. When the distributor is installed, it needs to go in in a specific position so that the rotor is pointed at the #1 cylinder position when the #1 cylinder is at TDC. The ECU is trying to do a normal advance based on what the sensors are telling it, but if the distributor is out of position, the ECU can't compensate for that angle.

 

Next time i am at junk yard(im there often) i will grab another coil

Vacuum only fluctuates normally with rpm change or throttle change. It seems normal to me.

Distributor was set with voltmeter via the fsm method. I was in a hurry so it could be off by a few degrees but its pretty close. I do want to recheck it as i was in a hurry.

I strongly suspect an issue with the tps, it is in a certain spot in the throttle that this happens. It isnt affected by load or rpm but throttle position only. I have changed the tps out once with an unknown conditon one and even tested with a voltmeter but i still suspect it. Changing it with yet another here soon and if that doesnt do it i will start checking continuity to pcm on my sensors.

Not sure what else to do.

 

Third tps shows same symptoms so its not that.

 

Those tps units were fairly robust and didn't malfunction very often, but if you feel it could be that, you could pick one up at the junk yard and see if there's any difference. If it's the same, I'd suspect it wasn't the tps - two of them wouldn't malfunction in the same range.

I don't have any manuals on the 5.2 and would be interested in seeing the write-up on installing a distributor with a voltmeter. If you have a digital version of this manual and can copy at least that part of the file (probably a pdf), let me know & I'll give you my email address to send it. The distributor should also have some mechanical setup procedure - this should also be in that Factory Service Manual (all the Mopar OBD1 engines I've worked with had only this mechanical setup - there was nothing specified that could be done with a voltmeter), did you follow that procedure? You mentioned earlier that it idles advanced 10 degrees and at 2500 rpm, the advance is about 30 degrees - this sounds excessive, does your FSM give specs on spark advance? If it's firing too soon, it will still idle reasonably but won't have much pulling power, and that's going to happen at roughly the same rpm and as long as the valves are closed and tight, you won't get any backfire in either direction. If you've ever worked with a carb'd engine where you stuck in the distributor in a specified manner and then adjusted the advance/retard condition, you could move the distributor around and get the same condition as you're describing. You may be 1 tooth advanced on your distributor or your timing chains could be off - you would need to do all the setup from a guaranteed Top Dead Center on cylinder #1.

BTW, I forgot to ask, you mentioned that you felt the engine was running lean - what color are the spark plugs when you pull them out?