High Idle

[jpyolo]

Hopefully I can find some help here. I am trying to get my 97 Lesabre running right. I was having intermittent stalling and upon testing figured out I had low fuelpressure. I replaced the regulator withslight improvement and then moved on to the fuel pump. I replaced the pump and changed the fuel filter and when starting it up I now have great fuel pressure. Once started though, the idle immediately climbs. Within about 8 seconds it is 3/4 wide open. Dropping it in gear quickly avoids this but not completely. The idle is high enough to make it jump when letting off the break at a stop. Eventually it will randomly stop while driving. Maybe 20 mins into my drive. Otherwise it runs great. I tried to assess the problem and I have a pending EGR Pintle Position code so I replaced the EGR but no help. Out of a guess, I unplugged the TPS while it was idling high like this and the idle dropped down and the engine was idling nice and low and sounded good. Plug it back in and immediate sky rocket of throttle. I replaced the connector but no help. I back probed and during high idle the voltage is at/around 4.11 which is obviously high for idle but I can't figure out why. Other parts replaced: MFS, PCM, Pcv, smoke test with no vac leaks. I just want to be clear that before replacing fuel pump there was nothing like this. The rough idle from before fuel pump replacement was a low shaky idle when first starting and would eventually go away after 30 secs or so if I could keep it running. Any help is appreciated. I have been blessed with this vehicle and would really like to keep it going.
Thanks!

 

[HotZ28]

Have you checked the voltage from the TPS from idle to WOT using the probe method, or on a scanner?

How To Test The GM 3.8L Throttle Position Sensor (TPS)

GM 3.8L Index of Articles

easyautodiagnostics.com

 

[Homer Jay]

Disconnect the Mass Air Flow (MAF) Sensor and see if performance improves.

 

[jpyolo]

I don't have the ability on my scanner but I have back probed it during operation. As soon as the car starts, the voltage on the signal wire goes straight to 4.11 and the engine rpms follow suit. The reference voltage has stayed at 4.86. The only other thing I figured out was while I had the TPS disconnected. The moment I unplug it the rpms drop to a normal level. While it was unplugged, I checked the signal wire on the connector and the moment I come into contact with both the signal and the ground the idle jumps right back up like it was. So this is happening with the TPS disconnected. I will try to disconnect maf when I get home and update with results. Thanks.

 

[jpyolo]

Ok. So I started it up and it immediately started idling high. I had everything plugged in and then I unplugged the MAF sensor and no change. I then unplugged the TPS as well and the idle dropped but it stumbled a bit. I plugged the MAF back in while still leaving the TPS unplugged and back to a stable low idle.

 

[HotZ28]

It sounds like you need a new TPS!

 

[jpyolo]

It sounds like you need a new TPS!

It is new. I even swapped it for another new one claiming it was defective and still the same result. I ended up returning that one as well and going with the old one. I wish it was the tps.

 

[HotZ28]

Wiring problem, or the PCM has failed????

 

[Homer Jay]

Another suggestion; Clean the throttle body and IAC (idle air controller). Does not cost much and recommended maintenance.

 

[HotZ28]

Are you absolutely sure the TPS was installed correctly?

 

[jpyolo]

I am sure the tps is installed right. The iac is new but I can clean the throttle body. It was cleaned last July but it may have already gotten dirty. The wiring was replaced for the tps a few weeks ago. The pcm is a newly refurbished one flashed to the car's vin. All of this started after replacing the fuel pump and filter. I went from having about 20 lbs of fuel pressure to right within the specified range but this started at the same time. What I cant wrap my head around is why the connector causes this to happen even with it unplugged from the tps but only if I am probing for voltage on it. Like I stated previously, when I test the signal voltage it shows 4.11 and immediately starts the high idle once I touch the meter to the slot in the connector. And this is with the connector off so I dont understand what else would be regulating the idle like this with the tps disconnected. I am on my last nerve with this. Thanks for all the help so far though. I will clean the throttle body and report back.

 

[jpyolo]

Bump

 

[HotZ28]

The PCM regulates the idle/rpm's based on feedback from the TPS. If you are reading 4.11v on the signal wire, that info is translated by the PCM as being at WOT!

 

[jpyolo]

So I assume I wasted my time and money replacing the pcm then. So if the 4.11 is telling the pcm WOT then what is setting the 4.11? Or is it the other way around?

 

[jpyolo]

Bump. Still struggling to find the issue with this. I am able to drive the car somewhat but I'm worried about the hard shifts into and out of gear with the idle being so high. Thanks.

 

[HotZ28]

Where did you get the PCM? Either it is non-functional, or the sensors are not communicating correctly, or you have a wiring problem!! Have you got insurance and a fuel leak with a lighter, that could help resolve this problem?

 

[jpyolo]

Well it is a salvage title with liability so that would only present more problems. I did another vacuum test with no leaks. I wonder though if I was having a leak at one of the quick connect fittings that attach to the fuel pump if that would cause this to happen. Thinking if the return line was losing pressure if it would over compensate or something. I'm not sure. It's just a guess at this point but that's what we've come to. I would have to drop the whole tank again which would suck but it is the only place I haven't been able to rule out and it was after swapping the fuel pump that the issues began. I am thinking of dropping the tank a bit to where I can reach under it to the connections and starting the car while I wiggle things and see if the high idle stops. If I can't find it after that I may just resort to dropping it off at a shop. I am just worried they are going to assume all of the things I have already replaced, some of which were very expensive. Maybe I can tell them to confirm with me before changing anything so that I would get a chance to tell them it has already been changed. I will try to keep posting as I fumble through this. Any more help would certainly be appreciated.

 

[Merkava_4]

If you backprobe the signal wire at the TPS, you should read about 0.5V at idle, on up to about 4.5V at full throttle. Also, the voltage at the signal wire is supposed to gradually increase as you turn the throttle by hand; you can do this check with the key on engine off.

 

[jpyolo]

Also, I got the PCM from Flagship One. They seem legit and they program the PCM before sending it after you give them your VIN.

 

[jpyolo]

I have back probed it and that's where I am getting the 4.1 at no throttle. I will make sure to try again with the engine off ans post my results. Thanks for the input.