Tuesday, July 03, 2007

Why me?

I must have angered the Car Gods or something. Another odd problem rolls into our shop for me to solve. Sometimes this job is more about fixing people then fixing cars.

One fine morning a 96' Chevy S-10 pickup rolls in with the complaint from the owner being, "after it warms up it bucks and jerks, doesn't seem to be getting fuel." This was a first time customer so I went threw a more then normal list of questions.

"When did the problem start? Has it gotten worse since it started? Has the "Check Engine" light illuminated?" And the most important question of all, "What work has been done previously on the truck and what have you tried to do to solve the problem?"

The answers where, "A week ago, no, yes it's on after a while and no I haven't done anything to it."

OK, I took it for a ride and it definitely had a "buck" to it and a "jerking under load". I stuffed it in a bay and popped the hood for a visual. Inside the engine bay sat a 2.2L four cylinder engine with a 5 speed manual trans going towards the rear gear. Pretty basic, straight forward truck to work on. I although had never ran into one acting in such a riding bull manor. My next step was to pull the codes since the Check Engine light did come on but only momentarily. I also wanted to check the Data Stream and some other parts of the PCM's brain. With it running at idle everything seemed normal. The codes indicated a P1406 for a EGR Valve fault. My first thought was I might have one sticking open since the GM EGR's from this era where prone to failure. I refrained from going knee deep into testing the EGR since I haven't felt one act this horribly under load but not affect the idle. Most GM EGR's I've encountered had some severe idle problems or just carboned up passages.

So off on another test drive to see what I could capture with the scanner hooked up. At speed and under load I would get a TPS code (P0122) which made me really wonder what was up. A TPS failure could definately cause these issues I had. I just couldn't figure out how the code would be intermittent so often. It would come and go on the scanner instead of being a hard fault. I also noticed at times while the problem occurred that the scanner would show the TPS dropping out. With that I thought I was onto something.

Back in the shop I grabbed my trusty Snappy Wave Form viewer and scoped the TPS and EGR. The EGR and TPS where dropping in and out under load but at idle where fine and doing some checks with the scanner I was not going to condemn either of them. I started to think a PCM with a internal short but decided to go talk with the customer again since this truck had some odd issues I wanted the OK to have it overnight and think this one threw.

Once I talked with him my whole approach to the job changed. Once I mentioned some expensive items may be bad he mentioned. "Dang it, I just had to drop a bunch of money on a new transmission and clutch." Whoa...what? You just put a new trans and clutch in? I thought nothing had been done to the truck lately?"

"Yes, me and my uncle did it, and the problem started not long after that. I asked him if the clutch job could have caused it but he said no so I didn't mention it."

With that info I told him I was going to do some more testing. Back at the shop I put the truck in the air and gave the underside a very good look over. And I am glad I did because when I looked at the driver side up near the exhaust manifold I found what you see in the pictures above. The electrical harness that runs from the passenger side, just above the trans, and up towards the driver side of the engine had fallen out of it's connectors and a section was resting on the back side of the exhaust manifold. Once I moved the harness I could see that part of the wiring had rubbed threw or burned off and where grounding out on the exhaust manifold. It looks as though when the trans was going back in the harness was pushed aside some and could rest against the backside of the manifold. The wires burned off? The two that had burned threw and where touching the manifold where the signal for the TPS and signal for the EGR....

Oh brother. I kick myself for not finding this earlier. I should have given a much more thorough visual inspection but I didn't because my faults where with parts on the top end and I really had no reason to raise the truck up. From the top I couldn't see the harness had fallen out. The A/C lines partially covered the area and frankly, I wasn't looking for something like that because my initial info lead me to believe no one had there hands in it all ready.

I was glad to find it, and really didn't spend to much time on the truck. My test drives and testing before my second conversation where only about 45 minutes. Once I knew what was wrong I had the truck back to smooth operation in about 30 minutes. Some knew wiring, fix the harness connectors, and put it in a better location and it was out the door.


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