Engine miss: What am I missing?

stdyhand · Dec 16, 2011

I have an engine miss on 91 GT. It has been like this for a while because I haven't been able to find out the problem.

The car has recently had a fresh oil change, new plugs (standard autolites), plug wires (msd wires), new battery.

The TPS sensor is set to the correct voltage. The MAF appears to be good, if I unplug it the car will fall on its face. The IAC sensor is good, if I inplug it the idle is really bad but will stay running. I did pull it off and clean it though just to see if it would help but it did not.

The fuel pressure is at 39 psi with the vacuum off and around 31-32 with vacuum. All plugs are firing.

I did a compression check and all cylinders are around 130+ PSI which isn't too bad for such an old engine that doesn't appear to have ever had any gaskets replaced. All cylinders are really close to each other though, none off by more than maybe 5 psi if that.

Manifold vacuum is at 18 inHG.

I am at a loss as to what could be causing this engine miss.

Any ideas what I could check that I might have missed?

Noobz347 · Dec 16, 2011

Did you do all of that to try and solve the problem or did the miss occur after doing all of that?

stdyhand · Dec 16, 2011

Noobz347 said:
Did you do all of that to try and solve the problem or did the miss occur after doing all of that?

I did all that to try and find the cause of the engine miss. It was doing it before I did all that. I just can't think of anything else.

The only thing I haven't done is replace all the gaskets. I think I am going to replace the head, intake, and exhaust gaskets, just to make sure there isn't a gasket leak causing the problem.

Noobz347 · Dec 16, 2011

A leak wouldn't show up like a miss.

How about the cap and rotor?

Noobz347 · Dec 16, 2011

...and

Have you pulled codes?

stdyhand · Dec 16, 2011

Noobz347 said:
...and

Have you pulled codes?

The cap and rotor is new along with the plugs and wires.

As for the codes, I can't pull the codes. I tried running a KOEO/KOER test but the EEC plug isn't putting out any codes. I covered this sometime back and I think the conclusion is either the wiring to the EEC plug is bad or the EEC itself isn't working properly.

I pulled the computer and I don't see any leaking capacitors or burn spots. But the J3 port has been sanded down so it looks like the previous owner either had a chip installed or hooked a quarterhorse to it once before. If that is case they may have disabled trouble code reporting.

Noobz347 · Dec 16, 2011

Did you do this code pulling with a scanner or using the home brew method?

jrichker · Dec 16, 2011

Computer will not go into diagnostic mode on 91-93 model 5.0 Mustangs

How it is supposed to work:
The grey/red wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned will work
properly. That will severely affect the car's performance. You will have hard starting, low power
and drivability problems. Since it is a dedicated ground, it passes through the computer on its
way to the computer main power ground that terminates at the battery pigtail ground. It should
read less than 1.5 ohms when measured from anyplace on the engine harness with the battery
pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector grey/red wire gets jumpered to power
which either burns up the wiring or burns the trace off the pc board inside the computer.
That trace connects pins 46 to pins 40 & 60. Only an experienced electronics technician can open
the computer up & repair the trace if it burns up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering
it to power can produce unknown results, including damage to the computer. The ohm test
simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground
(grey/red wire) on the self test connector and battery ground. You should see less than
1.5 ohms.

If that check fails, remove the passenger side kick panel and disconnect the computer
connector. There is a 10 MM bolt that holds it in place. Measure the resistance between
the grey/red wire and pin 46 on the computer wiring connector : it should be less than
1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the
computer is suspect. On the computer, measure the resistance between pin 46 and pins
40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal
ground has failed, and the computer needs to be replaced.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

If the first ground check was good, there are other wires to check. Measure the
resistance between the STI computer self test connector (red/white wire) and pin 48 on
the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer connector.

Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the grey/red wire goes. Almost every
sensor on the engine except the MAF is connected to it.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

See the graphic for the 10 pin connector circuit layout.

old_blue · Dec 17, 2011

can you describe what is happening? Is it all the time? is it intermittent? under WOT? idle?

stdyhand · Dec 17, 2011

The problem is only at idle. At WOT the car will really scream down the highway.

jrichker, that is exactly the info I needed. I am fixing to go out and check that. I sure hope I can find out if its the wiring or the ECM.

stdyhand · Dec 17, 2011

Ok well I did the tests above and apparently the problem is with the ECM. All wiring tests were good. The connection with the negative battery post and the computer signal ground was .08. The connection between the grey/red wire and pin 46 on ECM connector was .08.

When I connected pin 46 with pins 40 & 60 on the ECM itself, I got zero ohms. There was no connectivity and if I am understanding the instructions properly there should have been a connection because doing that test jumpers the pins together causing them to be connected, IF the connection inside the ECM itself is solid which apparently it is not.

I am going to pull the ECM case back off and see if I can find that burn connection again. I may have overlooked it before.

stdyhand · Dec 17, 2011

Turns out I was testing the ECM backward. I finally got it tested properly. The OHM's between pins 46 and 40/60 is 9.8 OHMS. According to Jrichker it should be 1.5ohms or less so the ECM is bad. The wiring tested good.

On one hand it sucks I have to replace the ECM since it costs more, but on the other hand I am glad it isn't wiring. I hate tracing out wiring problems

stdyhand · Dec 17, 2011

One last question, if the ECM's internal ground has burnt out and I believe after doing the tests that it has, what effect would this have on the engine's performance?

Would it only stop me from receiving trouble codes or could it be effecting the other sensors as well and causing my engine miss?

The engine runs fine other than the miss at idle. It idles around 700. Other than the miss I can't think of any problems. It isn't bad on fuel and many of the sensors seem to be working. If I unplug them the effect on the engines idle is obvious right away.

Engine miss: What am I missing?

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