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94 mustang gt 5.0 runs real rich. HELP

10K views 19 replies 6 participants last post by  wbrockstar 
#1 ·
Ok engine has an Edelbrock Performer RPM II upper and lower, Edelbrock 75mm throttle body, KN Cone air filter, stock MAF, BBK shortie headers, dual exhaust, stock(new) fuel pump, auto transmission. It has been idling rough and seemed to me like it shifted odd, really low rpms. Had a shop do some diagnostics and they said the O2 sensors were not switching and needed replaced. Brought it home, replaced O2s and ran good for about a week, then started idling odd, etc. so checked IAC and it was non functional and had a couple marginal OE(1994) plug wires, so new IAC, plugs and plug wires. It hasn't run right since, runs absolutely pig rich and won't rev up without stuttering. I found it had a Jet chip #79407 and thought that might be it. Took it out and no change, following some diagnosis tips tested MAF and got bad readings replaced it-no change. I have checked the FPR, EGR didn't think that would matter? all fine. I am at a loss. No check engine light when running, it does work when key turned on. Thanks for any help.
 
#2 ·
Try and either scan for codes with a paper clip stuck into the obd port and count how many times the check engine light flashes or get a cheap code reader. Obd1 doesn't always give a check engine light even if it has stored codes.
I'd probably ditch the jet chip.

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#3 · (Edited)
Scanned for codes. I got 116 and 636 which are coolant temp to low/high and trans oil temp to low/high. They are both cold cause it won't run enough to warm them up :frown: Thanks for chiming in I have removed the Jet chip it made no difference in how it runs.
 
#7 ·
Sometime back I had a bad engine coolant temp sensor in my 95. The motor would idle a little rough and the car would stumble at low rpms. Also the engine temp would get hotter faster.
They sensor is like $20. Don't get it confused with the temp sender that just tells the gauge what to read. http://m.autozone.com/engine-management/coolant-temperature-sensor
I saw the part where you removed the jet chip and it didn't make any difference, but I still probably wouldn't use it. What's it supposed to do, advance the timing?


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#4 ·
Is the conical filter open to the atmosphere or enclosed in an airbox??

To make sure the egr system isnt suspect,remove the vacuum hose from the egr valve,plug the hose then start the engine to see how its running.If it seems to be running better,take it for a test drive to see how it runs.
BTW- Im not sure if this makes a difference or not,but did you disconnect the neg battery cable for 5 minutes after you removed the chip?? If not,I would to make sure adaptive learning is starting from scratch,otherwise the ecm could be running off of values that were being used when the chip was in place.
 
#5 ·
Go through the checklist that wbrockstar has said. If all else fails check your ECT. its a major fueling component. It wont allow your car to enter closed loop, and the O2 sensors will never "switch." The computer will keep the car in a perpetual warm up mode. Test it like this

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.

Voltages may be measured across the ECT/ACT by probing the connector from the rear. Use care in doing it so that you don't damage the wiring or connector.

Here's the table :

68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
 
#8 ·
Sorry I didn't get back to you sooner, no service via cell. Maybe my description is lacking some, the engine will not idle and blows black smoke out the tailpipes from cold start, it stutters and will not run smooth at all. I have not tried to keep it running very long due to the horrible smoke and very rough running. It was suggested to disconnect the MAF and see how it runs, which I did and the car ran way better. So I checked power in and signal out on the MAF as instructed and came up with bad MAF. New one made zero difference in running. Is it possible the ECM is bad?
 
#9 ·
. Is it possible the ECM is bad?
Yeah its a possibility.You can remove the rear cover off the ecm & check the 3 blue capacitors & signal return circuit.The capacitors will be leaking,buldging or have broken/melted posts,if theyre bad.
Inspect the gold traces throughout the board for any burnt/open places.

Your symptoms sound like a bad maf,bp or ect sensor.It doesnt hurt to test the "new" maf sensor also,since parts come off the shelf "defective" at times.If youve got a multimeter that reads frequency,you can test the bp sensor.Have you tested the ect yet,like Krem suggested?? The ecm uses signals from the ect & act at cold start,to determine fuel mixture.Theyre also used for timing control during warmup.
Test the red wire,on all of the 3 wire sensors,to make sure theyve got 5 volts.That verifies the ecm is outputting the VREF voltage.
I would also test the signal wire & signal return wire for the ect,act & tps,after you test the sensors themselves.These are the main ones used during open loop mode.
 
#10 ·
Ok I'll check those Saturday. Sorry, but bp is ?? I have been out of town so not able to even look at it all week, then busy tomorrow also. Thanks for the help
 
#13 ·
Ok thanks. I was thinking about it and thought that might be it. To 89fox, it won't run well enough to try that, I had looked for a vacuum leak earlier before the really rough running and found none.
 
#15 ·
If you cant find a vacuum leak,like 89foxbody suggested,do more thorough tests for the maf circuit,listed next.

The following link details testing info for the maf sensor & its wiring.If you cant open/see the diagram in the link,the diagram posted below is the same one.The tests will determine if::
1)The "new" "remanufactured" maf you bought is defective
2)The maf harness itself has an issue
3)The ecm itself is suspect.
If all maf/harness tests pass,then that circuit can be totally ruled out as the cause.
Do these tests,even if maf codes are not present.

MAF Codes 26/56/66 Troubleshooting


http://www.stangnet.com/mustang-forums/attachments/mafplugs-gif.205136/
 
#16 ·
FOUND IT!!!! There was a blockage in the MAF sensing tube in the MAF body. Discovered it while testing for proper voltage change while running. No matter what RPM it always read like it was in ambient air. Took things apart and found sensor feed tube blocked, so it was not registering any air flow. Cleaned it out and voila! runs like a champ. Thanks to all who chimed in and offered advise and tips.:smile:
 
#17 ·
Thats great news.Good job man.
Your symptoms definitely pointed to a maf problem.I thought you replaced the maf sensor or did you return it & reinstall the oem sensor?? If its still the new/remanufactured sensor & it came off the shelf with the tube clogged, I'll definitely start inspecting any maf sensor I buy from now on.That definitely explains why the symptoms got worse,after you installed the new sensor.Your old sensor could've been dirty,causing the original symptoms to begin, then the new sensor caused the symptoms to escalate due to the clog.It sucks,that you replaced several components before locating the fault.The repair would have cost absolutely nothing,instead of $100+ dollars.
BTW- if youre running or intending on running a K&N or similar air filter that requires cleaning/re-oiling,be very careful about how much oil you spray the filter with.The oil will get blown onto the heated wires on the maf sensor & a film will develop,skewing the maf voltage signal.
 
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#18 ·
Thats great news.Good job man.

BTW- if youre running or intending on running a K&N or similar air filter that requires cleaning/re-oiling,be very careful about how much oil you spray the filter with.The oil will get blown onto the heated wires on the maf sensor & a film will develop,skewing the maf voltage signal.
I ruined one like that, cleaning didn't even help.
OP- What was it blocked with? Plastic, oil/grease, rat poop?



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#19 ·
The car was non running and partially apart for quite a while, part of that must have been with the air filter removed. A piece of blue shop towel was caught in the sensing tube of the MAF body maybe from cleaning? I do not remember when or why that was there.:frown: I just bought the replacement sensor not the whole MAF assembly it was not part of the new MAF.
 
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