MAP function when disconnected 86 GT (Page 1/1)
Superm1952 OCT 01, 06:26 PM
I have a high idle situation. With that said I have done a lot of reading etc. I see in more than a few instances that people disconnect the MAP to see if it changes the idle. I also read elsewhere that if you disconnect the MAP that the engine will shut off. In my situation the engine is at 2500 RPM and when I disconnect the MAP it shuts down immediately! Is this right or does this point to a possible MAP problem. New plugs, NEW IAC, .49 volts at TPS, installed new TPS, There are alot of things I can try according to these forums so off I will go but the real question is it normal for the MAP when disconnected to kill the engine. It is an old MAP. Thanks Skip in Fl.
Patrick OCT 01, 07:49 PM

quote
Originally posted by Superm1952:

I have a high idle situation.



With the 2.8, nine times out of ten, it's a cracked EGR tube which causes a high idle. The "jacket" on the tube often hides the crack.

Cover the end of your throttle body with the palm of your hand while the engine is running. If the engine doesn't stall immediately, you have a vacuum leak (which may or may not be the EGR tube).

[This message has been edited by Patrick (edited 10-01-2025).]

Mike in Sydney OCT 02, 12:53 AM

quote
Originally posted by Patrick:

With the 2.8, nine times out of ten, it's a cracked EGR tube which causes a high idle. The "jacket" on the tube often hides the crack.

Cover the end of your throttle body with the palm of your hand while the engine is running. If the engine doesn't stall immediately, you have a vacuum leak (which may or may not be the EGR tube).






Also, check the vacuum lines and connectors. The lines are plastic and the connectors are rubber. Chances are they are more than 35 years old and they get brittle with age and the heat from the engine bay.
Superm1952 OCT 02, 07:30 AM
Patrick, Will do that this morning. I didn't want to get into the meat of this .......since I was thinking I would get feedback to do my research on the forum. Anyhow since you were great in responding I will say that PO removed and blocked off the EGR. I am still questioning the MAP removal when the car is running for my own knowledge. Some articles say it still runs but there is a change in idle and other articles say it should die immediately since the ECM has no idea what to do and shuts the engine down. Have you ever tried this on yours? Thanks Skip
Patrick OCT 02, 12:47 PM

quote
Originally posted by Superm1952:

...PO removed and blocked off the EGR.



Well, that opens up another can of worms (with a potential lean condition)... but I still suspect you have a vacuum leak somewhere, even if it's not the (removed) EGR tube.

There's also a chance that the PO (who's obviously altered the factory setup) has messed around with the idle stop screw. It's not an idle adjustment screw. Is the factory cap still over the head of the screw, or is it missing?


quote
Originally posted by Superm1952:

I am still questioning the MAP removal when the car is running for my own knowledge. Some articles say it still runs but there is a change in idle and other articles say it should die immediately since the ECM has no idea what to do and shuts the engine down. Have you ever tried this on yours?



I have several Fieros, and when I was troubleshooting my '84 duke Here, electrically disconnecting the MAP sensor resulted in the engine running with a default richer setting. However, I seem to recall that disconnecting the MAP sensor on my Formula resulted in a non-running engine.

Skip, if your MAP sensor is bad, it should've set a code. Have you checked for stored codes?

[This message has been edited by Patrick (edited 10-02-2025).]

phonedawgz OCT 20, 04:37 PM
Swap the MAP sensor with your other Fiero. I doubt the MAP sensor is making it idle fast. Almost always the EGR feed tube is leaking on a 2.8. My guess is the guy who took it out left the feed tube there.

Cut a strip of aluminun can the width of the UPC symbol. Take the cap off the dist and lay it aside, wires still connected. Feel for the two 10mm headed bolts on the bottom of the snorkle. Loosen them and when loose, slide the aluminun block off test strip between the snorkle and the EGR feed tube. Tighten, reassemble and test. If the idle is reduced, your EGR feed tube is leaking.

[This message has been edited by phonedawgz (edited 10-20-2025).]

Dennis LaGrua OCT 21, 09:34 AM
The MAP sensor provides one.5-5V signal to the ECM that controls the fueling proportional to manifold vacuum. The O2 sensor provides a 0- I volt signal that influences fueling as does the TPS. If either burns out completely a code will be set. You probably need a full ECM scan.

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Vintage-Nut OCT 21, 10:01 AM
Wrong Thread - Sorry

[This message has been edited by Vintage-Nut (edited 10-21-2025).]