'91 Mustang 5.0 Engine will not accelerate past 2000 RPM

Gass-man · Oct 25, 2018

This all started when I replaced some deteriorating vacuum lines. Took it out for a spin a round the block and started missing... eventually (after about 4 miles) stalled and wouldn't start. Took it home and checked fuel pressure which turned out good. Checked the TMI module, checked good. Replaced dizzy and got it running again. I'm guessing the pickup took a dump. Timing was perfect, idle was perfect, checked TPS voltage & that was good. Ran fantastic for about 2 hours then started acting up. Would not rev past 2000 RPM. Stutter, miss, RPM drops to zero when I got to 2000 rpm. Changed spark plugs and wires at this time with little improvement, RPMs revved a little higher. Pulled KOEO codes initially got a 15 (ECM Memory) and an 18 (SPOUT out or wiring fault ). I've only gotten code 15 once. Checked the SPOUT and jumped with a paper clip to confirm that it was getting a connection. Finally got to code sequence 11/10/11. Pulled KOER codes and initially got 63 so I adjusted the TPS and code 20 for #2 Cylinder weak? Pulled KOER codes again and got 33, 44 & 94. Pulled the EGR... I don't think the EGR would have this dramatic of an effect on the performance of revving the engine, but I may be wrong. It looks like it needs to be replaced anyway, so I'll be replacing it tonight. Any recommendations are welcomed. Thanks!

I'll update this weekend after installing new EGR.

It's been running great up until now (3.5 years)
AOD to T5 conversion
High flow cats
New(er) O2 Sensors (within a couple of years)
New fuel filter
New Spark Plugs
New Dizzy
New Spark Plug wires
Run non-ethanol gas only

jrichker · Oct 26, 2018

Did you put thermal compound on the TFI when you replaced the distributor?

Code 18 - SPOUT out or wiring fault - look for short to ground in SPOUT wiring going
back to the computer. Possible bad TFI or defective 22 K resistor in the IDM wiring

Use a timing light to check the timing: remove the SPOUT and observe that the timing retards at least 4 degrees. Put the SPOUT back in place and observe that the spark advances at least 4 degrees.
This code can disable spark advance and reduce power and fuel economy.

Remove the passenger side kick panel and disconnect the computer connector.
There is a 10 MM bolt that holds it in place.
Disconnect the TFI module connector from the TFI and the measure the resistance between the yellow/lt green wire and ground.
You should see greater than 100 K (100000) ohms.
Check the resistance from Pin 4 on the computer connector (dark green/yellow) and the dark green/yellow wire on the TFI connector. You should see 20-24 K Ohms (20,000-24,0000 ohms).

Resistor location: A big thanks to liljoe07 for this information:

liljoe07 said:
Check over by the brake booster. Its not in the harness on the TFI, its on the main part of the harness over by the plugs that connect to the dash harness. About 6" or so from that, going back toward the EEC.

If I remember right, the resistor is covered in a shrink tubing that is sealed to the wires. So, you wont be able see any markings. The shrink tubing is labeled though. It's a 22kohm 1/2 watt resistor.

Here is the location.

Next measure the resistance between the yellow/lt green wire on the TFI module connector and Pin 36 on the computer connector. With the SPOUT plug in place, you should see less than 2 ohms.

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

This diagram is the wire side of the computer connector.

Diagram courtesy of Tmoss & Stang&2birds

Gass-man · Nov 2, 2018

Thanks [U]jrichker[/U]!

OK, haven't had a bunch of time to troubleshoot, but I did try to probe the wires for the 22k resistor (with no luck) and check the resistance from the #4 pin on the controller and the corresponding wire in the TFI connector... got an odd 6.4 ohm of resistance?!!!.

Re: the thermal compound on the TFI... it came with the distributor so I assumed that it was installed correctly.

As I mentioned previously, it ran flawlessly for 2 hours (after I had installed the new distributor).

I'll be troubleshooting tomorrow for a couple of hours.

Also curious if the EEC relay is starting to go, just a thought.

I'll post some pics too.

'91 Mustang 5.0 Engine will not accelerate past 2000 RPM

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'91 Mustang 5.0 Engine will not accelerate past 2000 RPM

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