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 PCM/ECU Going Bad, Maybe a Wandering Ground? Need input

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revmach
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Name : Jim
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PostSubject: PCM/ECU Going Bad, Maybe a Wandering Ground? Need input   Wed Dec 28, 2016 1:46 pm

Hi everybody,
I've lurked here forever and greatly appreciate the posted advice, diagrams, solutions and vehicle improvements. Things are starting to go a bit batty on my 98' Riv.

No modifications that would affect anything in this post, but here goes:

Electrical symptoms are multiplying and turning this daily driver (205,000 miles, mostly Michigan and Pennsylvania) into a less reliable ride. The following problems are sporadic, but do seem to occur more frequently in cold weather and don't seem to be affected by wet/humid weather.


  • Service engine soon light with the P0121 code (throttle position sensor insufficient activity).
  • High idle (as high as 2000 rpm dropping to 1500 as the engine warms),at startup but sometimes persisting for an entire 20 minute drive
  • Stall or hunt for idle after lifting off throttle
  • Delayed upshift, especially from 1-2
  • ABS/Traction light on for part or all of trip. Sometimes restarting after the engine is warm fixes this.
  • A slight blip of antilock engagement when first driving away (this could be something else entirely, but I feel and hear this little buzz and think I feel just a bit of brakeing; this is with no brake pedal engagement.
  • Delayed start (strong crank, but no quick fire). Fuel pressure is good, newish fuel filter, spark plugs, wires, etc.


 I do also have the cat, egr, o2, and evap errors blinking on my code scanner. I believe this is due to a recent and large exhaust leak that is ahead of the cat.

Related fixes have included: New throttle position sensor, new idle air control valve, cleaning mass air flow sensor, replacing all vacuum lines, replacing fuel filter, cleaned idle air passage and plunger.

As the title indicates, I'm leaning towards either the computer going glitchy or a wandering voltage / bad ground.  I could see a poor ground or wandering voltage be just enough to mess with sensors that are 5v based, without being enough to make any noticeable difference on the rest of the 12v system.

Any thoughts on how to proceed?

Thanks
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albertj
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Location : Finger Lakes of New York State
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Post Count : 6473
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PostSubject: Re: PCM/ECU Going Bad, Maybe a Wandering Ground? Need input   Thu Dec 29, 2016 11:17 am

revmach wrote:
Hi everybody,
I've lurked here forever and greatly appreciate the posted advice, diagrams, solutions and vehicle improvements. Things are starting to go a bit batty on my 98' Riv.

No modifications that would affect anything in this post, but here goes:

Electrical symptoms are multiplying and turning this daily driver (205,000 miles, mostly Michigan and Pennsylvania) into a less reliable ride. The following problems are sporadic, but do seem to occur more frequently in cold weather and don't seem to be affected by wet/humid weather.


  • Service engine soon light with the P0121 code (throttle position sensor insufficient activity).
  • High idle (as high as 2000 rpm dropping to 1500 as the engine warms),at startup but sometimes persisting for an entire 20 minute drive
  • Stall or hunt for idle after lifting off throttle
  • Delayed upshift, especially from 1-2
  • ABS/Traction light on for part or all of trip. Sometimes restarting after the engine is warm fixes this.
  • A slight blip of antilock engagement when first driving away (this could be something else entirely, but I feel and hear this little buzz and think I feel just a bit of brakeing; this is with no brake pedal engagement.
  • Delayed start (strong crank, but no quick fire). Fuel pressure is good, newish fuel filter, spark plugs, wires, etc.


 I do also have the cat, egr, o2, and evap errors blinking on my code scanner. I believe this is due to a recent and large exhaust leak that is ahead of the cat.

Related fixes have included: New throttle position sensor, new idle air control valve, cleaning mass air flow sensor, replacing all vacuum lines, replacing fuel filter, cleaned idle air passage and plunger.

As the title indicates, I'm leaning towards either the computer going glitchy or a wandering voltage / bad ground.  I could see a poor ground or wandering voltage be just enough to mess with sensors that are 5v based, without being enough to make any noticeable difference on the rest of the 12v system.

Any thoughts on how to proceed?

Thanks

1) check and clean the grounds. Critical grounds are in the engine compartment and under the back seat. There are grounds behind the kick panels near the front passengers - if you don't have water incursion these usually don't have issues. Also, clean the battery terminals.

2) Fix the exhaust leak, now. Downpipes (manifold to cat) are not that pricey new, the gaskets are even cheaper if that's all the problem is, and they are so heavy gauge that a junkyard DP should work.

3) your idle is high possibly (most likely) because with that exhaust leak the PCM can't figure out where to set it. Fix the leak. Warm idle in neutral should be around 700 RPM, IIRC.

4) Wandering idle might be the exhaust leak, might be the IAC passages are clogged or the IAC has failed, or that the throttle body needs to be removed and cleaned. HINT: If you replace the IAC without cleaning the IAC air passages and removing the throttle body and cleaning the back, the new IAC won't help really.

5) you have at least one bad wheel sensor. Don't use cheap wheel bearings especially for highway service. Ever. Given when the ABS engages I'd guess one of the front sensors is not sending signals consistently. When it fails the ABS engages then notices you are not braking and throws a code that turns off the ABS and turns on the light. Fix it now -- these cars are hard to drive in slick (fall/winter) conditions w/o traction control/ABS, depending somewhat on the driver. If you have or can borrow a Tech II the tool can tell you which ABS sender is crapping.

6) If your battery is weak (below 450 CCAs) all the sensors will go on strike so to speak and you get all sorts of codes. Might want to test that battery with a load tester.

7) the computers are probably OK but prolonged driving with them getting a diet of BS inputs and you will kill them off.

8) the transmission is electronically controlled, and the delayed upshift can be a result of the BS sensor inputs, in concert. Or it can be something else. Fix the other stuff first -- however, if your transmission fluid is dark or burnt (brownish and smells burnt) you need to replace fluid. IF (and only if) you need to change transmission fluid, consider switching to Dex VI but that will require a flush and you want to make absolutely sure the person doing it knows what they are doing.

Other thoughts guys?

Albertj
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revmach
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Name : Jim
Joined : 2012-08-28
Post Count : 2
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PostSubject: Re: PCM/ECU Going Bad, Maybe a Wandering Ground? Need input   Thu Dec 29, 2016 1:13 pm

Albertj

Thank you. To fill in a couple of blanks and to update regarding your input:

I got under the car last night and found the exhaust leak: 2/4 flange bolts have failed at the downpipe to cat interface.  These are temporarily clamped, which reduced most of the leak, but I'll need a new gasket and new bolts/studs. By itself, this did not change the SES or P0121 code.

Investegated positive and ground cables.  No corrosion, all tight except for a slightly loose positive battery terminal (1/8 turn to fully tight, no connector wobble beforehand). I'll double check ohms when I get a chance. Battery was at 12.25v, so it is on the charger now. I'll do a battery load test and alternator test when it is charged.

Alternator and battery are approximately 3 years/ 50k miles old, replaced as a set. Battery for sure is the OE spec AC Delco, and I'm pretty sure the alt was also AC Delco, definitely not a discount brand.

Cleaned the plugs and sockets on the TPS, IAC, and MAF.  There was no obvious corrosion in any of the connectors, nevertheless, this relieved the first four issues in my bullet point list.  Again, the problems are sporadic, so I can't yet guarantee this was a genuine solution.

On to the abs/traction issues.  I'll do a bearing diagnosis as soon as I can.  The front bearings have each been replaced in the last 10k miles (Timkens) and the hubs were properly torqued and retorqued, so it really shouldn't be those.  I also haven't had any of the typical abs alarm/light on cornering that I normally get when the bearings are going out. The rear bearings have never been replaced to my knowledge.

I did do some brake work in the fall: replaced a rear caliper, a rear hard brake line, and a front flex line.  The rear line exploded right after I bled the front hose, literally backing out of the shop. My mind was focused on the front, thinking I had air in the line. It took a half-bottle puddle of brake fluid and several attempts at re-bleeding the front before I realized the problem.  Anyway, I'm wondering if the brake/traction light could be related.  I have a firm pedal when I drive, but it does occasionally feel soft at startup. There are no brake fluid leaks, but is there maybe a check valve in the system that could be failing? If that is the case, maybe the pressure bleeds off when the vehicle sits. I think the physical blip of ABS predates any of this brake work.

I'll also check the abs wires and terminals and see if there are any issues there.

*Edits/Adds below*

Throttle body (including idle air passage) was thoroughly cleaned during installation of the metal LIM gaskets around 180k miles. Idle is rock solid and low when warmed up, looks like 650-675 on the tach.

Battery was topped off on the charger and is now steady at 12.86 volts (engine off) and passes the test on my scanner (Innova 3160e), I don't know how in depth this particular test is... Alternator output test also passes. I can't quite remember why, but the battery thermistor was replaced with the alternator and battery. I believe I was trouble shooting at the time because the parts store gave me the wrong alternator (it showed compatible, but wasn't, probably missing the higher voltage output Albertj mentioned).


Last edited by revmach on Thu Dec 29, 2016 6:48 pm; edited 1 time in total
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albertj
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PostSubject: Re: PCM/ECU Going Bad, Maybe a Wandering Ground? Need input   Thu Dec 29, 2016 2:02 pm

Thanks for the response, here's a bit more of what I know.

1) brake lines - at your mileage I'd think you 'd want to find an afternoon and replace all the hard lines except those that have no corrosion whatsoever - I think there will be a couple under the hood you don't have to mess with. At +/- 150,000 miles I replaced all mine with the PVC coated lines. I now have 300+K mi and no issues, although I might replace my soft lines again when I next have to do a brake job on my Riv.

2) traction light - probably unrelated. These cars have wheel sensors that are integral to the hubs and IMHO the thermal cycling eventually kills them. If you have good hubs (National, SKF, etc.) the sensor will die before the mechanical hub. Sucks.

3) Concerning the thermistor, you are not looking for corrosion on the positive cable. What GM did was to put a thermistor in circuit with the alternator field such that the alt voltage goes up pretty far when the battery is cold. The higher voltage accelerates the charge to compensate somewhat for these cars' relatively high (for the time parasitic load when "off" (they are never really off). 12.25v at the battery, disconnected, and I'd say your battery has life but is nearing its useful end and the test will show you're missing a few CCA.

4) Concerning the TPS, IAC and MAF: if the IAC air passages are constricted to any significant extent, the PCM can't set idle right. You have to pull the TB and clean the backside. Kind of like changing a diaper, you have to clean the kid's behind also... The TB gaskets are cheap, just do it. By the way, the easy way to tell you need TB maintenance is if the tach quivers or hunts at idle. Quivering = service it soon. Hunting = IAC air passages constricted, MAF dirty, time to clean them.
Until you clean them the problems will recur (soon). Been there done that.

5) checking the ABS wires is a good idea, check for breaks inside the insulation. However, I suspect a bad hub based on how you say it behaves.

6) Fix that exhaust leak for real. It's simple fix (although not easy due to location) and nothing but trouble if you have even a little leak. Again, been there done that.

Albertj
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Abaddon
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Name : Scott
Location : Macomb, Michigan
Joined : 2010-02-24
Post Count : 4150
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PostSubject: Re: PCM/ECU Going Bad, Maybe a Wandering Ground? Need input   Fri Dec 30, 2016 8:32 am

The buzzing noise when you first start the car and take off is nothing more than the ABS self test. I'm guessing this happens after every key cycle, and not every time you take off?
You may just be noticing it more now because the ABS pump is getting old and noisier?

Fix the EGR problem.....

The Traction light might be a transmission related concern. Certain Trans related errors will turn the Traction light on. Also, there may be an internal pressure error inside the BPMV. If you can remember, carry your scanner with you (if it reads ABS), and see what code(s) pops up when the lights come on.
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