35GT1

Everyone-

Need your expertise in troubleshooting a 2001 5.3L Vortec in a 97 Jeep Wrangler. The engine is doing some really strange things, specifically:

It idles fine, but when throttle is added, it will rev to around 1550 rpm, then start to stall, falling back to idle, then repeating the process as long as the throttle is held at any position beyond idle. It revs hard, then dies, then revs again, with the cycle lasting about 2 seconds each time. If you hold the throttle open, it will pop through the intake slightly. This occurs in gear or in neutral. It is not setting a check engine light, nor are there any codes in the PCM.

Engine/Chassis Specifics:

- 2001 5.3L Vortec from a K1500 GMC
- AEM Brute Force Intake System
- New MAF Sensor
- New MAP sensor
- New O2's (Primaries only)
- No Cats, dual 2.5" exhaust
- Painless wiring harness for the Vortec
- PCM reprogrammed by Arizona TPI (Recommended by Painless)
- New fuel pressure regulator (Delphi)
- New External Fuel pump 65 psi min, 85 psi max, 47 gph, in-line filter
- 3/8" line -6AN fittings
- Return through the old jeep feed line back to stock tank, with the stock jeep fuel regulator gutted out (per Advance Adapters Recommendation)
- Manual transmission (AX15)

I've hooked an OBDII reader to the computer. The TPS is working without any apparent jumps in resistance. One O2 sensor seems like it isn't reacting to engine changes. as the voltage remains fairly constant while the other one moves around more. MAF and MAP also seem to reacting correctly.
RPM information is coming from the PCM, and the coolant temp looks right. In short, the sensors, with the exception of the one O2 sensor seem to be working.

I've checked fuel pressure. It is a constant 50 psi with the vacuum hooked up and at idle. It jumps to 60 psi when the throttle is dumped. At no time does it drop below 50 psi.

I'm new to this forum, but I've been reading it a lot since starting this project and it's been a great help. Although not well versed on the LS1 style motors, I'm no rookie when it comes to cars or mechanics (I road race a 900 hp GT1 corvette and have degrees in mechanical and electrical engineering). I am completely stumped. and need some help.

Thanks for reading through all of this.

Speartech

iTrader: (14)

Based on the repetitive cycle of the problem, it almost sounds like a rev limiter is set wrong in the PCM. Since something looks wrong with one of the O2's, you might try unplugging both O2's from the harness and see if that has any affect on the problem.

I would say make sure the fuel pressure is staying up when you rev it, but it sounds like you already have that covered.

busta9876

iTrader: (3)

Possibly something missed when the PCM was reprogrammed. There is an paramater for Abuse Mode Test that stalls engine at 1600, then resumes at 1100. These are the stock settings....

now what is triggering the PCM to go into this abuse mode test I don't know. But that sounds like what it is.

Can you hold the RPM steady at 1200, 1300, 1400, 1500..but then as soon as 1600 hits it stalls back down to about 1100?


Good Luck.

35GT1

Speartechs' suggestion is a a good one. Before I had the exhaust installed, I ran the engine with open manifolds, and no O2s and it exhibited the same problem. Because of my limited knowlege of Vortec systems, I thought it may be due to the lack of O2's. I unplugged them again, and no joy.

The abuse mode may be a good lead, because it does exactly as busta describes. You can slowly rev it until it hits around 1550 rpm (as measured by the PCM), then it cuts out to an idle. If you hold the throttle in the same position, it will continuously repeat the cycle.

At one point, I though I may have a voltage drop at the PCM. I'm tapping the same ignition hot wire as the stock Jeep PCM (it's continuing to handle the housekeeping chores in the jeep like gauges, air bags, etc). I cut the GM feed wires (battery hot and ignition) and went directly to the battery. No joy there either. In retrospect I knew that wouldn't help because the OBDII reader was staying up during the cycle and a DMM wasn't seeing a drop but it was a shot.

I contacted the guys at Arizona TPI and they're looking at it too.

Thanks again for the help guys.

35GT1

An update for those of you that may be interested. Busta was right, the PCM was going into abuse mode test.

After talking with the programmer (Arizona TBI), they suspected the lack of a VSS input to the GM PCM may be the source of the problem. It seems that unless the Abuse Mode programming is removed, if the PCM doesn't see the VSS signal, it immediately goes into Abuse test mode. They were able to confirm this on a customer's truck in their shop by disconnecting the VSS.

In my case, the VSS from the original Jeep AX15 transmission is being fed into the original Jeep PCM so the stock speedometer would continue to work.

Anyway, they are reprogramming another PCM, in this case without abuse mode programming and sending it out. Hopefully that will be the end of this issue. Thanks again Busta for your suggestion.

This is a great forum.......

Sherk

iTrader: (6)

Sorry to bring up an old post... but I gotta say, this confirmed my problems!

In the past 8 months or so, I've had 2 engines in my truck, and they've both exhibited the same issues as 35GT1 was having. One was a Gen I 383 w/Vortec injection, and the other was a Gen III 6.0L. The only parts that were the same between the 2 were the computer, and the transmission (swapped the input shaft/midplate on the T-56 to LS1 stuff for the Gen III).

I had the transmission apart 2 times myself to do some minor upgrades, and then to convert to LS1... neither time did I pull the VSS. It was covered in metal sludge, and as soon as I cleaned it off, it started reading correctly.

It didn't throw any codes unfortunately, so I didn't have a damn clue as to what was wrong the entire time - I chased my tail a lot. Programming changes, a computer swap, cam sensor, crank sensor, nothing worked. Then I thought about it, and confirmed it here. Thanks guys!