MAP Sensor problems, PCM?

[Die Ford V8]

1994 firebird 3.4 v6 auto

map sensor throws a code of low reading high voltage. the mechanic we went to said to change the map sensor and it will fix the problem, but i already changed it.

i dont know what would cause the map sensor to read that,

i need a way to pull vacuum on it to test the map sensor as well.

the maintenance manual said if the map sensor tests good, that the PCM is bad.

how much for a new PCM and and how hard is it to change it.

car idles for a while and then slowly gets worse till it dies. seems to do ok at WOT, but stop at a stop light and your lucky if it stays idling. the fuel pressure tested ok, new fuel filter, new plugs and wires a few months ago. new o2 sensors a few months ago, new air filter. this stuff was just general maintenance, but the car has always had the map sensor problem. it wasnt until recently though that the car started acting like it does.

[94Redbird]

The PCM is simple to replace... just loosen two bolts, pull out, unplug the harness connectors, and plug up to the new one and pop it back in. A new computer is not going to be cheap so it might be more suitable to buy a used one on ebay or something. I have seen a couple on there lately for 3.4's.

Also if you decide to just toss the old one you should remove the knock module from inside because chances are it's still perfectly good and may come in handy or could even be sold to someone else who needs one. It is located behind the small plate on the computer case. The plate is held on with two hex screws.

[Die Ford V8]

i will be looking, but if you see any on the board or some other board please post up, as i dont have much time to search daily.

im going to replace the plug that goes into the map sensor, the map sensor again, and all the vacuum lines that are around it and attached to it. then i will test it and see if its the pcm.

[HBHRacing]

Why would you replace the PCM because of a MAP sensor code? Thats like replacing the motor when it's time for an oil change. totaly un needed.


The MAP sensor will send volatge to the PCM based on engine load. When it fails a rationality test (high or low voltage for the RPM being reported) it will set the code. Considering that you have a high voltage code I would number 1 start looking for a vacuum leak.

[Die Ford V8]

about the PCM being bad thing it was not me that suggested or brought up the thought, it was the HAYNES CAMARO MANUAL. anyways, like you said it seems very drastic to do.

the weird thing was that i changed the map sensor before and it didnt do a thing.
i put a new vacuum line on it with a new map sensor with a warrany and vualla.

the car runs tip top and doesnt studder. it has a 1% stumble if you know what i mean, but we turned the idle up 30 rpms or so and it works great.

thanks for feed back all.

the car is running great, 180,000 miles and counting!!! [img]graemlins/rock.gif[/img] [img]graemlins/rock.gif[/img] [img]graemlins/banana.gif[/img]

[v6guam]

Please... What is "studder" and what is "stumble"?
My 95 3.4 seems to studder a little if I'm not excellerating or coasting.

Thanks

[Die Ford V8]

im thinking it is stumbliing what your going through. hmmm.

stumbling that we have experienced with my friends car, was when we stopped at a stop light and it starts to miss a little bit. then a little more, when the ac would turn on the idle would get weaker. then after a while it would die.

studdering is when you hit the gas and it seems to skip a few rpms in the range. (insert sound effects here ;) ) almost like a light jerk throughout your rpm.

if your car studders it is most likely spark plugs and/or wires. chances are it isnt a coil pack, but you never know. fuel starvation would cause the car to slowly go through the rpms. it will show less power, but hesitation isnt usually related to fuel unless you are WOT and the fuel filter isnt able to let enough by so it "stumbles" until the fuel flow can get ahead of the rpm.

stumbling in our case was due to a bad map sensor and maybe a slight vacuum leak. the map sensor controls ignition timing depending on things such as altitude(atmostpheric pressure) and the position of the throttle.

another thing that causes studdering is a burnt valve. not as common, but always something to look at.

basically if you have good fuel injecting and good spark sparking on time, the only thing that can still be the culprit is vacuum issues or internals.


oh yeah, alot of the stumble our bird had before was because of bad o2 sensors. before it would run better with the o2 sensors unplugged they were so bad.

im sure im overlooking a few things.

what kind of problem does your car have, maybe we can fix the problem.

[v6guam]

I just installed a new fuel pump, 3 new coil packs. New plugs and wires. It has 110,000 miles on it and its strong as ever. I took out the cat and replaced it with a straight pipe. No inspection here on Guam. Other than a slight studder while cruising the only thing left to replace now are the injectors. After a long drive and the car sits for 15 min. the engine seems flooded on restart. Rich gas smell and the need to gun the engine a few times to clear it out. I'm thinking that one or more of the injectors is leaking when the car is off. If it sits overnight it starts up normally...What do you think? and thanks.

[HBHRacing]

Just so you know. the MAP sensor has no control on spark timing. It's used for several things. One is rationality tests for EGR diagnostics. but mostly is for fuel control. It will help the PCM adjust injector pulse width based on the load that it is seeing along with the MAF sensor. it will also "take over" if there is a fault with the MAF. It also helped to set up how much fuel to add at start up based on altitude.

[Die Ford V8]

i was just saying what it said in the manual about the timing ;)

[Die Ford V8]

this is my current problem,

read my posts thuroughly and i think injectors may be your problem. sticking open while priming and flooding your engine till it clears out.

http://www.camarov6.com/cgi-bin/ulti...c;f=1;t=015634

[HBHRacing]

a fuel pressure test would catch that. The pressure would drop quickly if that was the case.

[Die Ford V8]

Originally posted by HBHRacing:
a fuel pressure test would catch that. The pressure would drop quickly if that was the case.

the only bad thing is that it happens like once every 20 startups. im selling my car soon, so not too sure if i want to put a gauge yet.

[HBHRacing]

It's not a dash gauge. it's a hand held gauge used for diagnostics.

[v6guam]

"oh yeah, alot of the stumble our bird had before was because of bad o2 sensors. before it would run better with the o2 sensors unplugged they were so bad."

What happens to engine performance when the O2 sensors are unplugged?