fervernt

Ok, for a couple months I was having trouble with my car stalling out at slow speeds. I had no codes, and no luck figuring it out. well, I took her to the shop to get my fuel pressure tested, and the mechanic disconnected the MAP sensor to get to the schraeder valve. didn't unplug the wire, just unbolted it and pulled it outa the way. when the fuel pressure test was finished he just bolted it back on. When I drove away, there was no more running problem! I watched him do it, and he touched nothing else.
Well, in my brilliance, I decided "hey that's a good sign my sensor is bad!" so last night I went and bought a new MAP. well, lucky me, not only did it make nothing better, but now the running problem is back. tried putting the old map back on, problem's still there. jiggled wires around, nothing. looked for shorts in the wire up to the main loom. nothing. wire looks good. At that point I figured the plug itself was bad. but the voltmeter is reading 5v power, and 12v ground. I have NO CLUE what to do next. I'll probably hit up a scrap yard and get a new plug to see if there still might be something wrong with it, but I just don't know where else to look. any thoughts?
also, don't know if any of this could affect it, but my vacuum is only reading 14, my fuel pump is on it's way out, and I've got an exhaust leak I haven't gotten fixed yet. I doubt any of that is the problem, since I didn't touch any of that last night when I replaced MAP, but if it's possible, I'm open to suggestions.

fervernt

also, unplugged MAP, and tried again. code 34 comes up, that seems normal. but car still runs exactly the same with map plugged or unplugged. is that normal? I thought it would run differently if sensor was unplugged. leads me to think the computer is getting no signal from map whether its plugged in or not. I'll try testing that next.

fervernt

well, tested signal voltage. with key on engine off, my shop manual says voltage should be around 4.9. voltage is reading 3.7. my book says that the at 3.7, computer thinks it has 20 kpa. so my computer is thinking my engine has more vacuum than it should. I'm still not sure what could cause the signal wire to read less than it should though. I know it's not sensor, because new one reads 3.72, old one reads 3.77. could be bad comp, short in my wire, or bad plug as far as I can tell.

fervernt

replaced plug... nothing. still exact same problem. According to book, if it's under 4v it's either a short in the wire, or the computer is fried. I hope it's just a wire, but tracing that sucker is a huge pain. Im so frustrated right now, but at least it's getting narrowed down. If anyone can think of ANYTHING else, I'm all ears because I am not having much luck at all. I'm about to just cut the wire at the computer and run a new one to see what happens. but I might regret that decision lol

gus32

How did you fix your problem i'm having the same problem with my car on mine with the map sensor disconnected it runs fine but it shuts down when i make a complete stop but when i connect the map sensor its a different story i get ruff iddle and misfires i replace the sensor and same thing... i return the sensor thinking it was bad got another one and same thing... How did you resolve your problem..

fervernt

Haven't resolved it yet. I just narrowed it down to either a bad wire or malfunctioning computer. I'm leaning toward wire though because jiggling it around got it working for no apparent reason. that seems like a pretty good indication of shorting wire. Are you sure your map sensor is at fault? any codes?

ex-x-fire

What was your fuel pressure? This could be your problem, you said the pump is on its way out.

fervernt

last time I checked it, fuel pressure was good, primed to a good range, and ran in a good range. I cant remember what the numbers were lol but I looked in my shop manual and they were good. Haven't checked it recently, but I don't think that's what it is. I don't really know if my fuel pump is on it's way out, its only a month old, but it's whining and screaming like crazy. Gotta get it back to the shop to see what they got to say. It was running fine before I replaced the MAP sensor, and now is running bad. like a switch was flipped when I replaced the sensor. tried putting the old one back on and it doesn't change. I really gotta trace that wire and see what happens after that. I just hate working with wiring. its such a pain to get to.

ASE doc

It sounds like there is an issue in the harness. You just weren't lucky enough to find it in your check out. Follow through the trouble tree for code 34. It should instruct you to disconnect the ECM and MAP and perform tests to check for a short or open in the harness. Did you get your MAP voltage reading on a scan tool? Or back-probing at the ECM connector? If you got the reading on datastream, then you could verify that the ECM is the problem by back-probing at the connector and comparing the voltmeter and scan tool readings. Of course be sure that the ECM connector terminals are clean and tight.

fervernt

naw I don't have a code reader yet. too much money for me at this point. I don't think autozone loans out obd1 readers either. I should check though maybe I'm wrong. I just backprobed at the plug, power and ground were good, but only got 3.7 at the green middle wire. I'll grab my book and take a look at that code 34 test again.

ASE doc

A code reader for the OBDI cars was just a jumper wire in a fancy little case. You need a scan tool that can read ECM data to do any good. They are cheap used. Look on Craigs List for an old Snap On MT2500. They were called The Brick because of their shape and durability.

You're right that the the signal circuit should read 5v with the sensor unplugged and the key on. With the sensor connected, signal voltage will drop to 4.5 normally, with key on engine off(no vacuum). At v18 in hg, which is normal engine vacuum at idle, MAP signal voltage should be about 1.6v.

The 5v on the signal circuit is a diagnostic signal from the ECM and used by the ECM to determine an open circuit(code 34). With the connector unplugged from the sensor, you should see this 5v at the signal circuit(Lt Grn wire). If you are getting 3.7 on the signal wire in this state, try testing at the ECM(connector C3, pin F15). If you get 3.7v at the ECM, with the sensor unplugged, isolate the harness by disconnecting both the sensor and the ECM connector C3. Then test for a short between the signal circuit and ground. If the harness tests good and you are getting 3.7 on signal circuit with the sensor unplugged, it is most likely a failing diagnostic resistor in the MAP signal circuit of the ECM. You will need to replace the ECM. Unless you know someone who can repair it.

ASE doc

Something just occurred to me. What kind of voltmeter are you using? It has to have 10Megaohms input impedance to be used on electronic fuel injection systems. Many of the cheaper units are only 50K ohms and will produce false readings because the meter itself, when placed between the signal circuit and ground, acts as a voltage divider and pulls down the signal.

fervernt

http://www.walmart.com/ip/GE-Digital...ellow/17117739
I got this model from walmart. It hasn't been a problem thus far. when I tested the TPS it read in the correct range. Should this one work?
Also, my vacuum is only at 14 at idle, I think it's because of the altitude I'm at(Colorado Springs, 6000+ ft). Is that a legitimate assumption? I've been reading up on it and it sounds like vacuum should drop at higher altitudes, but the internet isn't always right lol.
Last thing is, if I find out that the wire is bad, should I go through the trouble of trying to find the short, or would it be better for me to just run a new wire from the computer and bypass the old one completely?

ASE doc

I can't tell from that link what the input impedance is. If it read the TPS correctly, that's a good sign. Higher altitudes have less oxygen density and will cause the engine to run on the rich side. This may lower idle vacuum as it affects idle quality. I have gone both ways on harness repairs. You can spend hours digging back into the harness to find the short, or just sister in a new wire. Be sure that any splices are clean and weather tight. That is a very sensitive circuit and as you are realizing, even a little resistance can affect drivability. Be sure before you jump to performing any repairs that you complete the testing to isolate and verify the cause

fervernt

Well, since I'm not sure if that voltohmeter is working like it should, I had a different idea to test. I want to unpin the MAP from the plug at the ECM, and see what happens. As I understand it, if MAP is unplugged, computer should use preset settings to run the engine instead and would set a code in the process. however, if there is a short in the wire itself, unplugging the sensor would still give the computer a false reading. So if I unpin at the ECM and eliminate the wire completely, it should run fairly well. If it
1) sets a code
and 2) runs at least somewhat better
I can assume it's the wire. If not then I could assume it is the computer itself. does this sound reasonable? or could I damage my ECM by unpinning the wire and running the car?

ASE doc

I can't say for sure what GM's strategy was on the SD cars, concerning default MAP function. I have never known an engine to run very well with the MAP unplugged. I would rather see you unpin the MAP to very carefully test voltage at the ECM MAP signal terminal with the harness disconnected. See if it reads 5v in this state. If it reads 5v with harness out of the loop and 3.7 with the harness connected, then you know you have a bad harness.

A quality DVOM, like one of the Fluke brand units, is available fairly cheap. Since you're doing your own diagnosis, I would say it's a worthwhile investment. A quality meter will have its input impedance spec printed in the user's manual. Also, look for the scan tool I mentioned on Craigs List. It will sell for about $250. With this tool you could quickly determine if your voltmeter is reading correctly by comparing your voltage readings with the actual MAP data PID in the ECM. You could also see now the MAP sensor fault is affecting the ECM's fuel trims. These are tools that anyone diagnosing an EFI car should own and they will never lose their value.

fervernt

Alright I'll keep an eye out for that stuff. I've been wondering what scan tool would be good for my car for awhile, so now that I've got a model I'll be keeping a close eye out for one. only one I saw on craigs right now is goin for $500. think I'll pass on that lol. I've heard of winALDL. is that worth getting? does it perform the same as a scan tool?

fervernt

Well, here's my results. came out nothing like you said they would... but I'm starting to lean towards bad computer now... Unplugged sensor and probed, gave me .03v. figured that didn't make sense, so I probed the ecm side of the harness. still only .03v. plugged sensor back in and probed ecm, went back up to 3.72! unpinned the wire to the ecm and probed the socket directly. still only .03. so it looks like something is fried in the ecm if it's supposed to be putting out 5v and is only putting out .03 right? any thoughts on what I might have done wrong before I go ahead and order a computer?

ASE doc

WINALDL will work but I believe you need to make a custom interface to connect to the OBDI(12pin ALDL) systems. I just looked again at the diagrams for the 91 and I may have been blowing smoke. It shows the diagnostic resistor going to ground. Which would cause you to see ground on the disconnected ECM pin. So, can't condemn the ECM from that. Sorry about that. That's not what I'm used to seeing, but oh well. Now that we know that, it may be that we are looking at a weak circuit on the 5v reference side. You have 5v on the circuit with the sensor disconnected but when you connect the sensor, it drops to 3.72. Try using a jumper wire to backprobe around the 5v reference circuit between the ECM and the sensor. See if the 5v holds with the sensor connected in this state. If so, you have found your bad wire.

If you can verify that the connection between the sensor and the reference circuit at the ECM is solid, and the voltage still drops to 3.72 with the sensor connected, you almost have to decide it's a bad ECM. It can't really be a short between ref and ground in the harness or you wouldn't see ground on the ground wire and 5v on the ref circuit. I suppose it could be a partial short to ground on the ref circuit. You can rule this out with a quick continuity test to ground on the isolated circuit. Isn't diagnostics fun?

fervernt

I hate diagnostics haha. Haven't had a chance to run those tests yet. With the fire here in Colorado Springs I haven't had much time to work on it. I'll update when I do to let ya know what I find out. I'm still about to the point of just replacing the damn wire to see if it fixes it. I know diagnostics are important, but christ they are a pain lol.

ASE doc

I keep a set of test leads in my diagnostics drawer with various terminals to fit most OE connectors and pins. I use these jumpers to perform quick tests by jumping around questionable circuits. I still pinpoint test switches and circuits before I perform any repairs but the jumpers help me shave alot of diag time. By the sound of things, you may also want to swap out the battery with one from another vehicle just to see if that might be the source of your trouble. As long as you're not buying parts to throw at it, there is nothing wrong with using known good test parts to narrow down the possibilities.

fervernt

Why would the battery affect the map sensor? It is a few years old, and I have had a few occasions where I've had to clean the connections because it builds up crap on it and wont start till I clean and retighten it. But I thought once the car started, it was run from the alternator?

ASE doc

Uhh, sorry. I confused you with another member who seems to be having low voltage issues when cranking.

fervernt

lol alright.

gus32

Did you resolve the problem on your car..???

fervernt

Alright well an update, hopefully the last before success. I'm close to my solution now I think, but I sure don't like how it's lookin. I went to the junkyard and found a complete wire with pin for the sensor, as I was sure that the wire was bad. Well I replaced the reference wire... nothing. still 3.72 volts at the ecm with the new wire in place. reference wire eliminated.
So I decided maybe the signal wire itself was somehow bad. spliced the reference wire back together and spliced the new wire into the 5v signal circuit. plugged it in and probed reference... nothing. still reading 3.72. signal wire eliminated.
I've already replaced the sensor and pigtail. So I'm pretty sure that I can eliminate everything except the computer. unless the 3rd wire(ground wire) just has magical properties that are designed just to **** with me until I drive this car off a cliff. So Doc, does this sound like I can safely say my computer is at fault now?

ASE doc

You can probably safely replace the ECM. Verify all powers and grounds just to be sure.

fervernt

Alright well I replaced the ECM now. It did fix a couple small problems I had been having, but unfortunately... MAP circuit still showing 3.72 volts. ECM eliminated. I literally only have two possibilities left.
1) my voltohmmeter is just special and doesn't work right. I think that's unlikely because it showed the correct voltage on the TPS when I tested that. However, I'm gonna see if I can find someone with a better one just to be sure.
2) there is something wrong with the ground wire. I don't know how that's possible with it reading 12v but I'll try testing that anyway. Those are the only options I really think I have left, so any other suggestions at this point would be GREATLY appreciated. I'm seriously starting to rethink my decision about owning a firebird at this point. I cant afford to keep throwing money at it like this with 0 gain from it.

fervernt

I don't have an o2 sensor on the car right now, at least not plugged in. Is there any possibility that could have an affect on the map sensor when it's not running? also, I believe I saw at one point that the ground wire at the MAP sensor doesn't go straight to ground. I think it might go through the CTS first but I'm not sure on that. Could that explain anything or give me any clues? I don't know what to try next.

ASE doc

The MAP shares a ground with the coolant temp sensor. The sensors use an ECM sensor specific ground. The 91 appears to have two ECM sensor ground circuits. Do you have the wiring diagram for the car? You should have diagrams both for the original chassis and for the 91 so that you know where the car started and what it should be now. Verify that the sensors are wired according to the 91 diagram.

O2 generally has no affect until the system goes into closed loop. The exception to this is if the sensor is shorted to ground. I have seen cases where a shorted O2 sensor circuit would cause the engine to run very rich immediately on startup.

fervernt

I THINK that I have eliminated the ground wire as a problem too. Not sure (my knowledge of wiring is still pretty limited) but what I did was stick a safety pin in as a backprobe into the ground wire on the plug, and ran an aligator clip from it directly to battery ground. figured if there was a break or something in the ground wire, adding a good ground would fix it. Still was putting out 3.72 though. no change at all. I actually got my o2 sensor working yesterday and that didn't fix my problem either. didn't think it would but I figured it was worth a shot. I've already seen an improvement in gas mileage from fixing that though. that never hurts haha

thirdgenBen

Lisle - Code Retriever
Part # 22700
Line: LIS

i got one at orellys for a few dollars

fervernt

Ben, thanks man but I don't think that code reader would do much good for me. I've already checked my codes, and there isn't anything for my map sensor coming up

Michael Cibelli

Well I'm having kinda the same problem my car cranks like the timing is off an when I slow down the tac goes down and almost stalls then jumps back up and goes down and up and down and it idles really low and it runs the exact same with the vacuum line going to the map sensor disconnected and the plug I'm really starting to thing the torque converter is causing it except for starti g problem

fervernt

Haven't had any time to play around with this recently, but I'm hoping I'll be able to take another crack at it again soon. Still don't know what I can do besides running the same tests over again but I've gotta get this figured out. It is driving me insane. Haven't had it stall out on me recently, but the problem is definitely still there. What I might do is try working on other things that could cause this to happen and see if I'm just completely off base as to what is causing it to try and die. I'm gonna try and clean up my IAC and throttle body for starters and see how it runs. Also putting in a different tranny because mine is on it's last legs now. (dark fluid and burnt smell, also when I leave the car in 3rd and coast, it sometimes jumps up around 500 rpms for some odd reason.) So I guess I'm looking for new ideas. If I didn't know my MAP voltage is wrong, what could cause it to try and stall at slow speeds, then jump back up to normal idle?

Jeffs82TA

iTrader: (1)

You should check those plastic plastic vacuum lines under the throttle body as well as others with a vacuum pump. My 1990 vette threw map codes when the vacuum line under the air breather port broke. Low engine vacuum will trigger a out of range voltage on a map system. Another thing that will cause a slow responding map sensor if the original small plastic vacuum line was replaced with larger vacuum line. You need to find out why you have low engine vacuum first whether it be timing related or cracked/leaking vacuum lines my 90 vette always pulls around 19 inches of vacuum and has never had a map code since.

fervernt

I'll recheck the vacuum lines again, but I've gone over them quite a few times with no luck finding a leak. also, the 3.7v i'm getting is with the car off, just the ignition on. As I understand it, it should be around 4.5v in that state. That's why I'm thinking its a wiring issue. also, because when I wiggled it around a bit (took it off to check my fuel pressure) the problem went away. Replaced the sensor and the problem came back. it seems that wiggling it around is causing the problems, which sounds like a wiring issue.

Jeffs82TA

iTrader: (1)

especially do vacuum test on the brake booster. any leaks can cause a map to wig out. WD40 works good in finding vacuum leaks just spray it around and if idle increases or decreases a leak is there.

RandyNJ

Anyone from this thread still around? Having the same MAP issue. Car runs with the map sensor out, but runs horrible with it in.

henryd3

iTrader: (128)

Don't know if it helps or not, but recently my car started to stumble and die when coming off the freeway. Barely managed to limp it home and next day got a code 33 which my
manual said was MAP sensor. Checked and swapped parts around but still ran really rich and poorly. Was trying to watch the injectors spraying and suddenly noticed my EGR valve stuck
wide open.
I imagine that messed with the manifold pressure quite a bit and after swapping in a spare EGR problem solved. Never had that happen before
so another learning experience!

RandyNJ

Mine was stuck open too and replaced...

henryd3

iTrader: (128)

So, did that solve your problem?

RandyNJ

No, it was replaced over a month ago.