What causes acceleration 2002 TB

[ragtopwife]

I am still trying to deal with this issue that I have had for a while now and thinking I am looking at it the wrong way.

Here is what is happening:

On start up the engine will rev to about 3000 RPMs and come down slowly to either 600 RPMS or 1000 RPMS, it's 50/50 on which it will settle at. The amount of time it takes to settle down to either is about 25 seconds. No check engine light. This is still in driveway in park.

Once I do put it in drive though the engine seems to be running fine and no there is no self acceleration.

I can drive it with no issues for 8 to 10 miles then the problem comes back. Comes back to the point that I can literally get on the interstate and let my foot off the accelerator and will continue to increase speed to about 65 MPH. But trying to drive on a secondary street where the limit is 45 I find myself constantly having to brake to slow the vehicle down.

Imagine this at a stop light in town driving and a vehicle that want's to accelerate on it's own. At a light I am at the point sometimes where I have to really hold down on the brake to stop the vehicle from moving. If I put it in neutral while coasting to the light the RPMs jump up to about 4000 and will take about 30 seconds to settle back to 1000, will not drop to 600.

Some of the time I will get a code. Sometimes I will not. The code that comes up is P0171 but to the best of my knowledge this might be coming up only cause I am trying to hold back the vehicle from accelerating with the brake.

I actually took this thing to Chevrolet and to my dismay those guys told they could not diagnose the problem till I have the engine harness replaced. Sorry but I have a hard time believing that.

Soooo here's my question:

What will cause the engine to accelerate on it's own? I mean is there something that tells the PCM to advance fuel and timing other than the throttle position or accelerator position? I would say the throttle body is bad but it has also been replaced and there is no change in my current condition.

If the position of the pedal was off should it not give me an error code?

Or as it could apply to the PCM as well could there be a bad sensor or something that is going bad bud not throwing a code and the problem is only intermittent but more so than not?

Thanks again for any insight.

I am trying to get my hands on a ELM327 so that I can record what is going on during these times and if anyone is able to look at it please let me know.

 

[Paul Bell]

P0171 is a lean condition.

Sounds like "false air", air entering the intake system AFTER the MAF sensor.

I would suggest you examine all your vacuum lines. Randomly plug them to see if the problem goes away. The brake booster and it's line could also be the issue.

Also have a look at the intake ducting and the throttle body mounting. Is the intake manifold properly bolted down to the heads?

 

[Texan]

P0171 System too lean (Bank 1)
How often and when does this code show up.
"Throttle body also replaced" What else replaced?
Could it be a vacuum leak? Loose intake manifold bolts?
How many miles?

 

[jeffro312]

I've had this same issue for quite sometime now I have elm Bluetooth dongle and torque pro not sure what to look for. I assumed it is just a vacuum leak. It's very random when it does it but at a light Rpms will be 1000 and in torque shows pending code same as yours. I'm able to clear the code and all is normal most the time for a while.

Only thing in torque I notice is that fuel trim for bank 1 sensor 2 is nothing just doesn't read any input not sure if that's something you can check. I just am too lazy and busy.

Also the last plugging vacuum lines suggestion isn't going to help as since there's a pending code will take time for the problem to go away won't be an instant thing. There's other ways check vacuum leaks. Basically car thinks it is getting to much air or not enough gas that's what lean means so I suppose checking fuel delivery might help. Fuel filter fuel pressure fuel injectors I'm no pro but good with rc cars and when they are lean means air fuel ratio is too much air or not enough gas making it think to much air. Anyway sorry for my rant hope I could help point in a direction. If I get mine fixed anytime soon I'll chime u in. Keep us update if you fix before me


Also what was dealer's explanation for replacing harneess to diagnose a lean code problem ??

 

[Paul Bell]

Also the last plugging vacuum lines suggestion isn't going to help as since there's a pending code will take time for the problem to go away won't be an instant thing.

No need to see if the code goes away. Plug the vacuum ports and see if the problem goes away.

As rubber lines age, they become porous and with engine movement, they can leak inconsistently. After a false air event, the PCM won't instantly return the idle to normal.


The dealer is shooting in the dark.

 

[Texan]

Pure speculation on my part. Could it be the fuel pressure regulator?
The problem is, you would need a fuel pressure gauge hooked up
when this happens to check.

Edit: but I still think a vacuum leak.

 

[MAY03LT]

I am trying to get my hands on a ELM327 so that I can record what is going on during these times and if anyone is able to look at it please let me know.

Yeah post them up. I took some screens of my elm showing idle speed at cold start up and exactly 2 minutes later. I missed the 3 minute pic it was right around 620 IIRC.

I'm surprised the dealer didn't check for pcm updates. I remember a few 02s that had similar issues where the pcm update was the fix.

edit - not for the 0171 but for the accelerating feeling.

 

[ragtopwife]

Thanks for the replies.

We did replace the fuel pressure regulator and have replaced the O2s.

Dealership was an interesting visit. They supposedly did confirm there was no vacuum leak but the reason for replacing the wiring harness was because of some butt connectors that were visible in the harness and not a "GM" splice. The other thing that was interesting was that I got to talk to the tech and asked him if there was anything that was reading incorrectly and he said no. I asked if there was a way to get a copy of the data and of course there wasn't.

The code P0171 does not come up all the time. I suspect that this is a false code or being triggered with the issue I am having but not related to fuel or timing issues.

The extra added air makes sense to me but I think it is happening at the throttle body itself and something is telling the blade to open up even more than it should without pedal input. I will try to confirm this once I get something that can read the TPS.

For the the time being, let's assume that there is no vacuum leak, and the fuel and ignition system are working properly. Could there be anything else?

Thanks again.

 

[littleblazer]

Question, when you start breaking does it jump and jitter some?

 

[Mooseman]

Check your intake manifold bolts. They are known to work loose and maybe with movement might give intermittent vacuum leaks.

 

[MAY03LT]

For the the time being, let's assume that there is no vacuum leak, and the fuel and ignition system are working properly. Could there be anything else?

For p0171 or for the accelerating? For the accelerating I'm still with checking that the pcm is calibrated to the latest software.

 

[ragtopwife]

When I am braking it is just as if I had my foot on the pedal but I don't.

Will check bolts shortly.

For the accelerating. Checking the PCM is calibrated to the latest software, is this a dealer only option?

 

[littleblazer]

Okay, my theory is out then. Sorry I couldn't help more. [emoji2]

 

[Texan]

What harness & circuits were the butt connectors used on, and
how many are there?

 

[ragtopwife]

Injector harness just past the connector toward the front of the vehicle. There's 6 connectors I think.

 

[MAY03LT]

It might not be dealer only. I know mooseman was able to check cal ID's and update with his new tech2.

 

[Mooseman]

If you can find someone with a Tech2 and TIS2000, it can be updated. Problem is finding someone in your area.

 

[ragtopwife]

Finally narrowed the problem down to a malfunctioning PCM. Is there any way to replace that without going to the dealership to reprogram VATS? my understanding is that vehicle wont start until reprogrammed... Any way to pre program or disable the anti theft altogether?

Thank you!

 

[Sparky]

You could possibly buy an entire PCM that is tuned from PCMofNC. They would have your VIN programmed into it and everything. Then all you'd need to do is the security relearn (takes a few minutes to do DIY, not hard), and then get a CASE relearn done.

At least that way you'd also benefit from a tune.

 

[Mooseman]

Heck, you could just buy a used PCM from a yard, same year and engine, install it yourself and do the security relearn yourself. The only thing you'll need is to find a shop with a Tech 2 scanner or a dealer to do the CASE relearn. Having the VIN programmed is not necessary and this would be your cheapest option.

 

[Paul Bell]

I've gone the ebay way for PCMs. They'll even set your VIN and stock settings for your truck.

 

[ragtopwife]

Follow Up:

New PCM did not fix the problem. Back to square one. The mechanic we had it at says that it could now be something related to the torque converter stator clutch and something about leaking pressure.

With that said. I have been looking around trying to find something in TSBs and cannot find anything related. He was using a website that had a lot more information (I am guessing a subscribed to site for auto mechanics) than just TSBs.

Would one of the devices like ELM327 be able to record data between the the PCM and trans? Would there be a way to diagnose a trans problem via one of these devices (electronically) or would pressure tests be the tell tale of a bad converter or leak? Wouldn't the the trans only operate by what the PCM tells the solenoids to do? There would be no data going back to the PCM from the trans correct?

Thanks again

 

[Mooseman]

Yes, Torque Pro should be able to do this. There are a few data streams including gear and TCC slippage %.