Humbled by a 2002 Jeep Grand Cherokee 4.7L (FIXED)

Life seems to have a way of humbling me, when I get too cocky. A customer came in complaining of a 2002 Jeep Grand Cherokee 4.7 that will run for a few minutes without issue, but once warm, it will backfire and die. He said that after it dies, the gauges "go crazy", there is a lot of "clicking" under the hood, and fuel sprays out of the air intake. The parts cannon has been fired. He gave me a list: New Idle Air Control, ECT sensor, new "computer screws", Coolant flush, Tranny service, Spark plugs, Ignition Coils, Starter, Battery, CKP sensor, water pump, thermostat, and more. He said that he heard I was the guy that could fix it. I told him that if I couldn't fix it, I wouldn't charge anything. This is my policy and have not, yet, had to let one go unfixed/unsolved.

But, this thing has been kicking my butt, for days. :dry: It is, no longer, about getting paid. It has become personal.

When it was first brought in (on a truck), it drove into the shop without problem. But, before I could get the scanner connected, it backfired and died. Attempting to restart was unsuccessful. Fuse 6, the 30 amp fuse for the ASD relay was blown. I put a new fuse in and restarted. After a few moments...Backfire and Die. I look at the fuse, which used to have nice, shiny solder points. It was not blown, but I could see the solder points, now, looked dull-grey and not smooth. All the while, the EVAP purge solenoid and injectors were clicking. Suddenly, fuel started spewing out of the throttle body. (It came in without an airbox. He said a backfire destroyed it.) I pulled Fuse 6 and it stopped. After clearing the flood, I put an amp clamp around the ASD relay LOAD circuit and restarted. When the backfire occurred, the amperage shot up to 42 amps (on a 30 amp fuse) and the wire became very warm. With a thermal imager, I found and repaired a voltage drop at splice (S128). Now, I restarted and, again, Backfire and Die. 40+ amps on a 30 amp circuit...wires cool. Coils hot to touch....fuel spewing out of throttle-body. Turned the key off. I wanted a way to leave the key on/fuse in and investigate the 40+ amp current draw without fuel spewing, so I removed the load from the ASD and applied it directly to battery positive, supplying power to the coils and injectors with key on or off. I restarted and soon enough, as expected...POW! Die. I turned the key off killing the fuel pump. A very noisy 40+ amp current draw. I disconnected the ignition coils, one-by-one, and watched the current draw drop about 2 amps with each disconnected coil. Injectors still clicking away.

The computer provides the grounds/control for the coils and injectors. The coils and injectors are being grounded when they shouldn't be. When doing voltage waveforms of the injectors, I noticed injector #5 was missing control. It look like this HAS to be a bad computer. I put in a new PCM. I, now, have control to Injector #5, but after a short warm up....POW! Die....:pinch:

This has all the indications of a bad ground/voltage drop. And, I HAVE found both bad grounds AND voltage drops. I have had about three different "Ah HA!" moments where I have found an issue and thought I found "THE" problem.

This Wave is of Bank 1 Injector voltage, before things go crazy.



Yellow is injector 1, then big a gap where injectors 8 and 4 fire on bank 2, then GREEN is injector 3, then a small gap where injector 6 fires on bank 2, then Blue is injector 5, and Red is injector 7. This is what I expect to see.


Then, after reviewing the .vsm, I noticed this, brief anomaly while the engine was running.



Notice the three consecutive blue and red events. They passed by, UN-noticed, as the pattern returned to normal..............Until.....POW! Die.


This is what it looks like when the injectors are going crazy..



I have been all over powers and Grounds. Why/How are the injectors and coils being energized? :blink:
The same problem, even after I "threw" a PCM at it.

scope the cam sensors and crank while the injectors are firing after the engine stalls see if there is a signal from any of them??

cheryl hartkorn wrote: scope the cam sensors and crank while the injectors are firing after the engine stalls see if there is a signal from any of them??

This. Sounds like the cam or crank sensors are going crazy.

I find this issue fascinating. I don't get why it takes a while for it to act up. Initially, I thought maybe is was an open loop/closed loop thing, but I have changed my mind.

It might have to do with heat and/or vibration. For example, I knew of a BMW X3 where the vehicle speed sensor would only fail when in reverse. The engine/transmission torquing would cause the sensor to fail.

I would check what the other two posts have suggested, cam and crank sensors. I'm guessing they are going crazy and making all the injectors fire at once causing the amps to go through the roof.

Please post what you find. This one has my curiosity. I so wish I could work with you on this...

Best of luck.

What tha... :blink: That's bananas! Serious question: How is fuel shooting out of the throttle body? Is the intake plenum filling with fuel or something?

Another vote for cam/crank, if you didn't go over it prior to PCM replacement. I think these engines will run without the CMP, so you could try leaving it out and see if the issue quits.

My other thought it a coil, or another output, 'spiking' the PCM and causing havoc. Two wire coils like this can short between windings and end up leaking secondary voltage into the primary circuit. Maybe try watching coil ramps, to see if one coil in particular is firing right before the stall?

EDIT: Never mind, coils have been replaced. :oops:

Spot on, Cheryl. Thank you, very much!

cheryl hartkorn wrote: scope the cam sensors and crank while the injectors are firing after the engine stalls see if there is a signal from any of them??

I automatically thought, "Bad ground", on this one. Then, finding voltage drops, hot wires, and disconnected ground straps, I got a case of tunnel vision. Watching SD's recent video on a bad body ground causing loss of com and other wierdness, amplified my tunnel vision, I believe.

Hind sight being 20/20, I can't believe I neglected to scope the CMP! I DID scope the CKP. :dry:

I'd like to believe that I, eventually, would have gotten around to it. :blush:

Thank you, again, Cheryl! My hat is off, to you. You pulled me out of my tunnel vision.

This is the CMP signal after the engine dies and injectors are going crazy.

Congrats on the victory

Well done. Cheryl totally called it. Now, go get paid!

I started thinking about a CKP/CMP problem from this thread:

www.scannerdanner.com/forum/post-your-re...wd.html?limitstart=0

This member had a VERY similar symptom, but was ultimately caused by a jumped timing chain. Point is, these PCMs react very strangely to timing issues. :blink:

Eric O of South main Auto had a similar issue. Going to the beginning of part 2 to see the cmp sensor signal on the scope. His was burning up the coils. So might keep that in mind

Part 1

Part 2

Tyler wrote: What tha... :blink: That's bananas! Serious question: How is fuel shooting out of the throttle body? Is the intake plenum filling with fuel or something?

It must have been DEEP, is all I can say. It was a fair amount spitting/spraying out the Idle Air port. It would take a few different attempts, 5-10 minutes between, with the accelerator pedal to the floor for "CLEAR FLOOD" mode, cranking for 15-20 secounds each, before it would fire. And by fire, I mean FIRE! Flame shooting out the throttle-body. Finally, when it would re-start, it would smoke the shop out with the thick, white smoke that accompanies liquid fuel in the exhaust system, until it was all burned up.

dont buy a p.o.s. aftermarket sensor or youll be doing it again in no time.... under warranty

bruce.oliver wrote: His was burning up the coils. So might keep that in mind

I burned up, and had to replace, 4 of the new coils it came in with. I walked away from it while it seemed to be calm and quite. Key was on. ASD circuit hard wired to the battery. I went into the brake room to review the .vsm file of the injectors. My helper came running in saying there was smoke coming from the engine. Fortunately, no fuel was being sprayed. Four of the ignition coils were totally melted/destroyed. That was my fault. I walked away with things "ON".

cheryl hartkorn wrote: dont buy a p.o.s. aftermarket sensor or youll be doing it again in no time.... under warranty

I did. It was my only option, late Saturday afternoon. At least its not Duralast.

Tyler wrote: Well done. Cheryl totally called it. Now, go get paid!

I started thinking about a CKP/CMP problem from this thread:

www.scannerdanner.com/forum/post-your-re...wd.html?limitstart=0

This member had a VERY similar symptom, but was ultimately caused by a jumped timing chain. Point is, these PCMs react very strangely to timing issues. :blink:

Very similar, indeed. Scanner Danner posted in that thread, too.

ScannerDanner wrote: Nice Tyler! Thank you.
To the OP, are there any other driveability complaints with this? These are known for the cam sensors to wig out and cause multiple random coil firings with just the key on! Coils will melt, fuse will blow etc.

:woohoo:

And, NOW, it's known by ME!
If, only, I had searched this Forum, in the beginning. It seems like such a long time, ago.... :S

pole71 wrote: And, NOW, it's known by ME!
If, only, I had searched this Forum, in the beginning. It seems like such a long time, ago.... :S

:lol: I still forget that we HAVE a search function at times, since we didn't for so long. We're accumulating enough threads now that doing a search may be worthwhile for new members. :huh:

Not just new members.