Hi,
I've got an '84 SE which I bought of my brother in-law about 5 months ago. It drove like a dog when I got it, so I replaced the usual stuff - filters, spark plugs, vacuum hoses.
Did all this, and the car seemed to drive a lot better, but then I noticed that the MAP sensor was unplugged, so I plugged it back in.
Now if when I drive it with the MAP sensor plugged in it feels really sluggish on acceleration and the exhaust makes popping noises when I release the accelerator.
Has anybody got any ideas as to why its doing this, its very humiliating being overtaken by Ford Fiestas on the Dual Carriageway...!

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Matt ffolliott-Powell

Hey Matt!
You may want to reconnect the MAP, then disconnect the battery for a couple of minutes to reset the ECM...
By resetting the ECM, you will go back to the factory defaults in the computer and the car will reprogram with the ECM hooked up properly...
There are a few criteria to be met before the ECM calibrates to the new stuff, Temp, and speed are the big ones, so if you can get on a freeway for a leasurely drive over 50 MPH for about 10 to 15 minutes, the car will settle down to the new readings...
It should drive much better!

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crash... The Fierowrecker

Thanks Fierowrecker, will give it a try...

This forum's really great! Thanks for the quick response!

MATT!! WELCOME TO THE FORUM!!!!!

With the vacuum hose off of the map sensor, the ecm thinks that the engine is running wide open so it gives it more gas. If it runs better with the vac line off, then you have fuel problems....may be as simple as a clogged fuel filter. Hard to say without more info, but it's definitely a fuel related problem.

Phil

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GTDude
25 years GM experience

There was probably a reason it was disconnected...its probably bad! Its probably the easiest part to replace too! Be sure to get a GM replacement.

Welcome to the Fourm!

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Its never going to be finished...

Visit my website www.purplefiero.com for Show Dates and Fiero Stuff!

Thanx for your replys!

To answer some of your suggestions, I did think that it could have been a faulty MAP so I bought a new one and the same thing happened.

Got a new fuel filter as part of my initial service.

The vacuum hose was connected to both MAP's, although the original hose was cracked so I replaced it with a second hand one from the Fiero store.
One question though, does the hose from the MAP sensor to the TB have to be of a certain length?

It should be the same lenght and diameter as the original. If you have to replace it with a rubber hose it should be as close as possible the the original and as short as possile. If it is to different in size it will affect the vacuum reading to the computer.

Other than fuel related as mentioned above...

Check the ignition timing. Make sure the diagnostic terminal is jumpered when you do it (the same as when you get the codes).

Without the MAP input the ECM goes into limp mode and doesn't change the fuel or the timing the same way.

DRH....I'm afraid you're incorrect on this one. MAP sensor does not put ecm into backup mode. It gives the ecm a full rich signal or about 4v. The only way that his car can run better with the map sensor unplugged is if he has a major vacuum leak someplace or it is running very lean for another reason. Many possibilities. I suggest you take it to a shop and have them hook a scanner up to it to find out what the real problem is. Make sure you ask questions and take it to a service facility that is familiar with GM computer systems. At any rate...your problem is a very lean condition..for what ever reason. Hope this helps ya.

Phil

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GTDude
25 years GM experience

GTDude,

There might be some confusion on what he means by unplugged. I thought he meant the electrical connector which should make the MAP line go to 0 volts and the ECM would recognize that as bad input and go to backup mode. You're probably thinking the vacuum line which would make the ECM think WOT. It should still set a code 33 but I'm not sure about backup mode then...

Either way, the main spark advance table in the ECM is based on RPM and Vacuum, if it's not getting a signal from the MAP sensor the timing advance will be different than if it were getting the signal.

Thanks again for all you help guys,

I reset the computer last night after reconnecting the MAP and took it out for a drive but the car was bucking and jerking all over the place at low revs, if I put my foot hard on the accelerator it would eventually stop bucking and accelerate normally, but again if I changed down a gear back to low revs it would start bucking again. Gave it a good drive to see if the computer would sort it out, but it just continued...it almost feels like the fuel isn't getting into the engine properly - it splutters.

Had to disconnect the MAP again eventually so I can at least drive it without feeling like I'm a contestant in a Rodeo.

Is it likely that it's a vacuum leak somewhere or is it an electrical problem?

"The vacuum hose was connected to both MAP's, although the original hose was cracked so
I replaced it with a second hand one from the Fiero store."

What do you mean "both MAP's" ?

What is the condition of the spark plug wires, distributor cap and rotor, and ignition coil? What replacement spark plugs did you use? Have to ask...

Could this be a case where the EGR valve is partially stuck open?

Have you check the timing? Compression test?

Condition of the catalytic converter?

Through the process of elimintion, whatever is left, MUST be the problem. Unless it is a systemic failure involving multiple systems.

when you get a replacement, look inside where the electrical connector goes and see what color it is. (red,orange,black,blue,green) you have to replace with same color to work properly, there each different. My local GM dealers only listed the black one, which didnt work in my car, even after I modified the connector to fit. I found the full selection at Autozone and worked perfectly. (after about 5 hours of driving around to find proper one) My 86 SE chassis used an orange one. With the black one, that dealer said was 'right one', ran just like you said.

[This message has been edited by rogergarrison (edited 02-15-2002).]

DRH.......communication failure as usual.....LOL. Sorry.

Phil

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GTDude
25 years GM experience

Matt, it is possible you did not drive it far enough. I know they say you need to drive it under normal conditions and only moderate acceleration, for the ecm to relearn, but after replacing a bad O2 sensor, I had to drive mine about 20 miles for it to stop the bucking & hesitation. Maybe I just have a dumb ecm tho. Mine is also a 84-4cyl.
There should be only one MAP sensor on your car, unless GB uses something different than our Fieros use. I would look very closely at your egr system and vacumn lines in general.
And the vapor canister vacum lines. Some of these 84's were actually built in late 83 I think, and the components maybe even before that, as the Fiero used components from other model lines, so they are easily approaching 20 yrs of age.
Welcome aboard.
Don

quote Originally posted by englishmatt: Thanks again for all you help guys, I reset the computer last night after reconnecting the MAP and took it out for a drive but the car was bucking and jerking all over the place at low revs, if I put my foot hard on the accelerator it would eventually stop bucking and accelerate normally, but again if I changed down a gear back to low revs it would start bucking again. Gave it a good drive to see if the computer would sort it out, but it just continued...it almost feels like the fuel isn't getting into the engine properly - it splutters. Had to disconnect the MAP again eventually so I can at least drive it without feeling like I'm a contestant in a Rodeo. Is it likely that it's a vacuum leak somewhere or is it an electrical problem?