Pontiac Grand Prix GTP 1998 Service Engine Soon Light Help

[Maverick41]

Folks,

I am looking for a little help with my GTP, which has a 'service engine soon' light on, and can't smog. The car has 115,000 miles and has been well maintained.

The car runs beautifully with the exception that it did stall and quit at a stop light once since the light has been on (light has been on for about 30-60 days or so).

The problems all started when I took it to my local Jiffy Lube, for an oil change, cooling system flush, new radiator cap and the fuel injection cleaner added to the tank. I knew that was a big mistake as soon as I saw that the high school kid working on the service, was the same knuckle-dragging, trogladyte that recently closed the hood on my mini van after the oil change, and failed to remove and store the hood strut properly, which folded it in half.

Soon afterwards, I got the 'service engine soon' light that came on. I just figured that someone accidently knocked some hose off or possibly a connector while they were monkeying around under the hood.
Apparently not. I recently took it in to my local smog certified gas station to have the 'service engine soon' light checked into and fixed. Well, a week later (without the car), and a couple of hundred bucks into it, we are not much closer to solving the problem than when we started and the mechanics seemed perplexed.

When they first got the car in, I told them about the Jiffy Lube servicing issue and also told them that when I took it to Jiffy Lube, the car would start instantaneously when you hit the key, hot or cold. Now it cranks for about 5-7 seconds unless hot and just shut off. Then it starts quickly. Also told them that I had replaced the fuel cap with a Napa locking cap about that time (key issue). Since that time, I have replaced the Napa fuel cap with the Pontiac OEM cap just to be sure. The Napa cap had a different vent arrangement to it and it raised some eyebrows.

The local smog station stuck the sensor up the tailpipe, and told me that I had 4 error codes stored in there. Three for Evap and one for the crankshaft sensor. The codes had been in there for two years, which was the time that I purchased the car, and the previous owner had had it smogged (different facility than my regular guy). Also, we discovered that there is a mystery wire, that apparently runs from the fuel tank area, up to the PCM. It is a loose wire that someone slapped in, and runs out of the wiring harness, up along the frame rail. My smog guy called the previous smog guy and talked to him about it, and it turns out that there was some sort of electrical short from two years ago, and rather than rip out and put in a whole new harness, they did a one wire fix which seemed to work, and the wire is well terminated, so I have no problem with the patch fix.

They cleared the codes and tested the car and the 'service engine soon' light would not come back on, for them. Well, I took the car and drove it for three blocks, and it popped back on, so back it went.

Now the local smog shop is grasping at straws. They wanted to test some of the sensors first, before replacing the very expensive PCM, needlessly, which suited me just fine. They did so and even pulled the tank to test the fuel tank pressure sensor, which they say is ok. They say all of the sensors that could be effecting the problem 'appear' to be ok.

Now I just got the car back, and asked for a list of the error codes. Seems that we now have 5 error codes:

PO135
PO336
PO446
P1646
P1665

I am assuming that they are the three original Evap codes, on crank sensor code, and one mystery code we can ID. So now we are to the point that the local smog mechanic is suggesting a replacement of the PCM, which they say appears to be a little bit 'burnt', but not crispy .. LOL

Also, since the 'service engine soon' light has been on, the tach occassionally goes to zero while driving and the TCS shuts off and displays the OFF on the button.

I have no problem replacing the PCM if it is needed, I just don't want to replace the thing and see that the light is still on for something that they missed. The car runs great. I recently took a trip in it of 1,100 miles and it got 28.5 mpg. The only thing it does do everyonce in a while, is when it is climbing a hill and the superchareger is in a light boost condition, the car has a light surge to it, which goes away if I hit the Power Shift to on; no biggie.

If anybody has had a similar problem, I would greatly appreciate hearing about your ideas here. I am doing this on a budget and can't afford to throw thousands at the problem, which is the Dealer's approach, replacing anything and everything until the problem goes away. Hell, I can do that! LOL

Also, if anybody has any information on rebuilt PCM's vs new, I would appreciate the input, and who specifically to steer clear of.

Thanks,

Maverick

[BNaylor]

Welcome to AF.

Wow, quite a laundry list and history. "PCM, which they say appears to be a little bit 'burnt', but not crispy"??? What do the two 80 pin electrical connectors at the PCM module look like? PCM is possible but maybe not.

From what I see the priority issue is the P0336 DTC which is a crank position sensor range-performance issue. The 24X pulses are abnormal. Based on the tach dropping to zero rpms and the TRAC OFF light this is consistent with a bad crank position sensor.

The P0135 DTC is an 02 sensor heater circuit malfunction. This is the one before the CAT convertor. Could be a bad 02 sensor or missing the heater reference voltage from the PCM module.

P0446 and P1665 indicates a problem with the evaporative emissions vent circuit. Either a bad vent solenoid/valve or related wiring/electrical connector.

P1646 appears to be a fluke DTC and I see no listing if it for a '98 GTP.

However, from the general listing I have for possible extended GM DTCs P1646 is also related to the evap vent circuit output.