TPS wiring questions

[bigalpha]

Alright, i went out and disco'd vacuum lines and plugged them. Nothing changed. I hooked up the vacuum gauge to the lines on the harness that sticks up from the manifold and the vacuum read in the middle of the 'normal' area with hardly any movement.

There are no noticeable cracks or flaws in the manifold that I can see. Everything looks like it's screwed in nice and tight.

Do you think it's possible that it could be a fubar ground to the ECM?

 

[kastein]

Could be. If the engine block is at a higher voltage than the ECU ground due to wacky grounding problems, the TPS voltage sent to the ECU will be interpreted as if the throttle was further open.

 

[bigalpha]

Where does the ECM ground at, the engine block at the dipstick?

 

[kastein]

I've got no idea actually, that's a good question. I know some stuff grounds to the driver inner fender on 97+ but I don't know RENIX from a hole in the ground. If it grounds to the engine block, my last post could be pretty worthless... depending on how the ground failed.

 

[Shorty]

Where does the ECM ground at, the engine block at the dipstick?

si, senor-- pull the 1/2" nut and clean everything 'til it shines-- should be big battery neg. and two (I think) eyelets for ECU and engine sensor grounds. I ran an auxiliary ground from the dipstick tube to the ground strap at the cylinder head and back to the battery to ensure complete grounding-- all exits open, no waiting.

 

[bigalpha]

I put on a new 5-90 batt neg cable already and cleaned up that particular spot. Tested with a DMM, it tests out good.

 

[PaulJ]

I think you should try to determine where the air is getting in that is allowing it to idle so fast. I'll bet it's the IAC that's allowing it to pass but you need to positively determine that. Put your thumb over the IAC port on top of the throttle body and see if that drops the idle significantly. You can also check the idle port on the top left side of the TB the same way. You can even cover the whole top of the throttle body with your had. If you do find that the problem is related to the IAC allowing too much air to pass, I think the most likely scenario is that the ECU is not controlling it properly. A bad coolant temp sensor isn't going to have a lot to do with this. The TPS has a huge influence and you've only been testing the TCU half of the sensor. The TCU half only influences shift points. I had a lot of problems with my '87 recently and found that the TCU half of the TPS worked fine but the ECU half of it was dead. I also found that the return had about 3-4 ohms to ground. One of the splices in the harness was bad near the firewall. You can check from the TPS connector to the battery neg. post. Be aware that the high input impedance of the DMM can often confuse the readings when you're checking for ground. I also checked for the TPS signal at the ECU to make sure I had the proper signal there. This was really I how I found out that it wasn't getting a good TPS signal.

 

[bigalpha]

I covered the whole throttle body with my hand and it only dropped the idle down by about half. I assume this means it HAS to be a vacuum leak; and a sizable one at that.

 

[kastein]

Definitely. If you block off the intake completely, you should either get spit out the tailpipe or the engine should all but die.

 

[bigalpha]

Thinking maybe the intake manifold?

 

[kastein]

You already tried blocking off the vacuum line ports on it one at a time, so it's either more than one vacuum line section or the intake manifold, yeah. Personally I'd bet on either the intake manifold gasket or the throttle body gasket.

 

[bigalpha]

I suppose I could try the water trick on the TB mating surface, too.

 

[bigalpha]

:looser::looser::looser:

Before, when I adjusted the throttle plate, I adjusted too far open. I adjusted again, this time with the throttle plate just barely off of the throttle body wall. Still idles a wee bit high in P, but I haven't put all the hoses back and the air filter box, etc.

Boy, do I feel like a complete and utter schmuck.

:doh: :doh:

 

[kastein]

Well, at least your MJ has stopped acting like a Toyota.

 

[bigalpha]

Indeed - thanks for everyone's help.

 

[joe_peters]

Indeed - thanks for everyone's help.

Glad you got it!

Don't forget to recheck the TPS adjustment after adjusting the throttle plate.

 

[bigalpha]

Glad you got it!

Don't forget to recheck the TPS adjustment after adjusting the throttle plate.

Ah, thanks for the reminder - perhaps this will solve the remaining 800RPM high idle.