I've been having some weird issues with the idle in my car for a while now and I think I finally found the cause.

Here are the symptoms:

Idles fine when I first start it up (although the idle screw is all the way against the stop when in 'idle set mode', it does idle at 750 which is close to factory spec). After I drive it for 15-20 minutes it won't idle back down to 750, but stops at about 1050-1100. If I turn the car off and restart it at this point it will idle at 750 until I hit the gas again.

When the car is cold the idle and WOT test and reset fine all the way back to the LH/EZ at the harness. I had replaced the TPS with a new one right after I did the top end refresh last winter and it was metering fine EVERY time.

I had checked the cable and throttle operation. Everything tested good in the garage cold or even 'warm'. The difference was apparently that there is warm and then there's WARMED UP. I was testing everything in the garage with it just idling and revving it up just a bit to clear things out. What I should have done was go for a long run of at least 20 minutes, then returned home and started working on it.

Apparently the idle switch doesn't click back into position when the car is at full temp (including the oil) even though it tests fine cold. It is amazingly consistent, which is why I had such a problem finding it I think. It was resetting EVERY single time I shut it off or tested it cold or even warm. It was not resetting almost every time it had been running for a long time so the only way to find it would have been to monitor it during or immediately after a run and I just never checked it then.

I was finally able to verify this with the data logger using Ken's Bridge-it. I could have also checked the timing after a long run, but I didn't think there would be a difference between when the temp gauge says it's at full temp and fully wam. (The way to verify your idle switch with a timing gun is: If idle switch is reset it will come down to about 10 deg., if it hasn't reset it stays in cruise mode at about 20 deg., WOT jumps to about 40+)

So I guess my TPS needs to be adjusted just a hair. I put in some allen screws instead of the phillips-head screws that were on there. The allen screws are the same size as the CO adjustment screw: 5mm. I'm hoping I'll be able to get the CO adjustment tool in there and loosen it enough to move it slightly, then get enough on it to tighten it down. I don't want to have to pull the intakes again.

 

Mike,

Would loosening the throttle cable just a bit have any effect?

 

Or adjusting the throttle linkage rod. If it's too tight, perhaps the engine is expanding just enough to trip the idle switch.

 

If loosening the throttle cable doesn't fix this, it's probably a TPS switch that's starting to fail. I've seen this on Bosch TPS switches on other cars; sometimes you will hear the click when leaving the stop, but no click when coming back to it.

Dan
'91 928GT S/C 475hp/460lb.ft

 

I'll take a look at it tomorrow night when I get home, wasn't able to verify whether the inability to reset is internal to the switch or not.
I'll try popping that off first, and if the switch resets when the car is at full temp I'll see if it's the rod or the cable.
The TPS is new. It might be going bad, but I think it's an adjustment because it does reset when it's cold or when I shut the car off and restart it hot. (and it makes good contact, 20 ohms or so for the whole circuit)

Thanks everyone, I'll confirm whether or not it's inside or outside the TPS in the next day or two and post the results. I'm curious if anyone else has had this happen to them.

 

OK, I checked on this over the weekend and I'm 99% sure I'm going to have to pull the intake runners on that side and adjust the TPS.

I pulled the rod off of the throttle console and revved it and let it fall back into position and it still doesn't reset the TPS every time. It was tighter when hot (I think) so I backed it off two turns and it does reset the TPS every once in a while, but its still not resetting every time when the car if fully warmed up. This means it's not the throttle cable or the rod.

Unless I can snake the 5mm allen wrench in there and adjust it in situ, the intake will have to come off.

While I'm at it, I'll make sure the throttle plate is getting fully seated because I have that thing where the car idles up when you put it in idle set mode, not down and the idle set screw is all the way against the stop.

The worst part is that the car is running incredibly well other than a slightly higher than normal idle when fully warmed up (950-1000rpms) , I just want it to be perfect.

 

I seem to remember being able to remove/replace and adjust the throttle switch on my former '86 by just removing the air box, AMM and center T-pipe and leaving the actual intake in place.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Dan,

I tried that once and didn't meet with any success, but I'm game for trying again. Maybe I can just maneuver the lower air guide enough to be able to get to the switch screws.

Thanks for the tip, I'll take a shot at it.

 

Mike
I do not have any way to test the TPS other than listening for the "click" when I manualy open/close the throttle. After a long drive over the weekend I shut it off and could hear the TPS click open/close when I moved the throttle very slightly open/closed. This is the same outcome when the car is cold.
I have been sucessful in accesing the TPS by removing the side intake bellow and then removing the center T intake. This is how I replaced the TPS about a year ago.

 

If you pull the outter plenums off, and the airbox off then you can loosen the clamps and remove the big pipe which connects the side boxes to the TB. You can then use a ratchet with a screwdriver bit to change it.

 

Thanks, I guess I didn't try very hard the last time.

There are a couple of other ways to verify that the TPS is resetting (both will verify that the LH is 'reading it' as well):
1) Using a timing gun: if your idle is about 10 deg. at idle, the idle switch is resetting, if it's over 20 deg., it's not
2) Using a meter on the LH plug (not sure which pins right now, but it's in the test plan) will confirm that the switch is good and that TPS plug and wiring are all good back to the LH.

I guess I'll be going diving this weekend to see if I can get just a hair of adjustment on it, and also to verify the throttle plate is resetting all the way. I'll be really glad if I can do this without pulling any of the intake runners again!

Thanks for the confirmation Colin. I replaced the phillips screws that hold the TPS in place with allen screws the same size as the MAF adjuster screw. I just like allens better than phillips head. So Hopefully I'll be able to get in there with that.

 

Not sure if you checked this, I once had an issue with the TPS, caused by dirt in the cable turning point. When hot the assembly became lazy in turning back, leaving the idle switch open. I removed the cable assembly, cleaned it properly and lubricated, and put it back again. Worked fine ever after.
Also check your gas pedal to see if it moves freely.

regards
Theo

 

Thanks for chiming in Theo. I'll go back through the whole thing. I think it must be on the throttle/switch side since I popped the throttle rod off and it still didn't reset (when running at full temp).

That's part of the problem because I can clearly hear it clicking and it shows closed on the meter when I'm working on it cold, but apparently when it gets up to full operating temp something changes and it won't reset by itself (unless I shut it down).

 

Ok, today I played hookie from work to get my car fixed and was able to get the TPS adjusted properly.

I didn't have to pull any intake runners, just the airbox, MAF, intake side boxes, ISV and the top 'T' portion as well as the three 10mm nuts that hold the lower air guide in place.

Then I was able to maneuver the throttle and lower air guide up and over enough to get to the bottom TPS screw. I ended up putting it almost all the way to the stop, tested it and closed everything up again (no spare parts!)

Then I took it for a nice long 20 minute run to allow the engine to get up to full operating temp. The idle would drop back down to about 750 (which is where it was set last) every time and when I got home I confirmed it by checking the timing.

So the bottom line was that although the TPS was resetting perfectly cold or when it was just started and allowed to warm up at normal low RPMs, that wasn't the way it was acting when 'fully warm'.

The switch had been tripping (and audibly clicking) when the throttle rod would just start to move, but it was before the actual throttle plate even moved. Now it clicks only after the throttle plate begins to open about 1/16th of an inch and that seems to be working well.

 

Mike
Great work in tracking this down. This is very helpfull for us '85 '86' owners as this is something I think a lot of us experience at one time or another.