1998 6C: Erratic tach and stumbling

Noticed this getting worse in the last week or so... intermittently the tach will jump up and down, sometimes all the way up to 8k, then back down, then back up, etc, and obviouslythe engine is not making these rapid shifts. Then, sometimes the tach is seeming to work normally, but the motor will stumble or surge a bit. Quite often, the tach needle just sits on the peg at zero, but obviously it's more likely at about 1200 or so doing 25-30mph. The speedo and other gages appear unaffected.

This seems like an electrical short or something. Does anybody have any areas to investigate for a damaged harness or something? Is there some sort of checklist I can follow? Could the tach problem be affecting the engine stumbling?

Thanks,
Tom

I brought the van to a dealer (Noonan Pontiac) in Sterling heights MI. Dealer checks out the SES light first, and reports to me that:

"Well, it says Crank sensor bad, Cam pos sensor bad, Evap vent solenoid bad, and PCM shorting out. Plus, there's obviously a leak in the intake manifold. We didn't even check on the tach sensor problem yet.

Evan solenoid - $210
PCM - $520
Intake manoifold - $1200

I'd suggest that we cut your losses. Why don't you just come in for a trade in?"

I am insulted! Rather than fix the problems, they want me to trade it in! I had the intake manifold replaced 30k ago (at the same dealer), and they want me to replace it again!?!

Dealer also says PCM is a "dealer only" fix, since it "needs to be reprogrammed while it is on the vehicle, and only dealers have the ODBII programming equipment to do it". Sounds fishy to me.

Just FYI.
Tom

I'd say you have one problem that is causing all the codes to show up. One system will affect another, in my experience. You are right to be insulted! Don't get cheated to boot!

And you don't need a $1200 intake manifold if you have that all-too-common coolant leak. You DO need a new gasket set, but that don't add up to $1200! :mad:

Take the van to another GM dealer, they will have the same equipment.

"I suggest that we cut your losses...." That ain't what they're trying to cut, IMO!

One of the codes that showed up was shorted eval solenoid. $210. When I questioned their quoted 2 HOURS service reapir time, I asked him what justified 2 hours to replace a $20 (dealer price to boot) eval solenoid... " I think they've got to drop the tank or something"

The result for my $92 "diagnostic fee"? A poor photocopy of a han dwritten note from a technician, showing a few trouble codes. When I asked about getting a printout of the diagnostics, the service guy says "that's all we got. the tech just put in the system "customer declined service"". What a rip!

So what's next? For a little more than the $92 they raped you for, you can get a code reader and have done that yourself!

Do you have any intentions of trying to troubleshoot this one yourself? I bought the factory service manuals from Helm, Inc. for the wife's van. Spent $100 or so, but for two phone-book-size manuals with all the info, it's well worth it!!

There are people on this site that may be able to help you out too; Flatrater comes to mind, along with one other (GMGoodwrench?) that I can't remember the user name for sure.

Or are you going to try finding a good shop?

Good luck with it, and keep us posted!

Of course I am going to do the work myself after this fiasco... Funny, it seems like after getting jobbed by a few shops here-and-there, I seem to get the idea that the dealership SHOULD be the best place for me to go... Perhaps, but I can't stand to deal with the ignorant arrogant sales/service people who tell you that their part/service is the ONLY way you can repair your vehicle. "Oh by the way, replacement would probably be cheaper than repair of your 6 year old van, I mean, it's already got over 100k miles on it"! What a crock, what a racket! That's only 1 year after a "normal" 5 yr financing program, and it's dead/unservicable 1 year later!

Part of the story they told me was that a new PCM (no such thing as a reconditioned unit) must be reinstalled and reprogrammed by them on site or it'll never work. New software and such. Dealer-only tools, etc, etc, etc. Repair cost $530. New drive belt you say, looks worn? Repair cost $120. What, new intake manifold gasket too? Another $1200?!

I just emailed I guy I found on ebay. $250 for a preprogrammed PCM, matched to my vehicle content via part number and VIN, Guaranteed to perform perfectly. Latest software installed. $240 shipped in less that 1 week. That beats the dealer by nearly $200, and results 2 weeks faster.

Last night I pulled the PCM leads for about 15 mins to clear the codes. And I cleaned and applied dielectric grease to all the fittings around the fuel pump and evap canister. All codes are gone now, and dash light is out. I've only run it for a few mins so far, but so far so good.

The tach is still dead though. If there are no PCM codes, yet the tach remains erratic and/or sometimes dead, I'm forced to conclude that the problem lies with either the gage or the "sending unit" wherever that is. Perhaps it's simply a signal sent from the crank sensor, but I'm not sure. I'll need to find one of those service manuals for a detailed wiring diagram.

Tom

Well, I fixed all kinds of stuff this weekend, and I've got some good ideas about most of the stuff that's not fixed yet...

Like above, the dealer tried to tell me "the PCM is shorted, you'll need to have it replaced and then have it reprogrammed. Nobody but the dealer can do it. PCM repair cost'll be $520, plus programming... Nobody except that dealer can do that, and it needs to be programmed while it's in the vehicle...

Long story short, I found some damaged wires in a bundle that goes through the floor right behind the driver seat. Apparently there was a problem with the connector some time in the past, so somebody ghetto fixed it by band sawing the connector in 1/2, twisting-and-taping all the wires back together, wrapping electrical tape around them, and then stuffing them back in the hole... The wire were all shorting together, many were corroded and wet, and it looked like the steel frame had cut through the wire bundle from all the chafing...

I recut all the wires back to good copper, resoldered the all, and shrink wrapped each of them individually. Then I shrink-wrapped the whole bundle with some of that strong-thick blue shrink, then I siliconed the whole thing back through the old connector body to give it a friendly surface against the metal body to prevent further chafing.

NO MORE CHECK ENGINE LIGHT!

Only one more code, eval canister solenoid circuit. I figure I'll need a new solenoid, that wet wire bundle mess probably fried the stupid thing. Anybody know how to replace it? Dealer says it's a 2 hour labor job... I know that's wrong (since I dropped the whole tank in less than that) but aside from that I'm clueless...

Thanks!
Tom