Tps Or Pcm Or What, Intermittent Gremlin

[TGIVinnie]

Recently purchased a 99 3.8 L from a woman my wife works with.

The car runs great when it is first started and for about 20 minutes, then it acts like the TPS is bad, i.e., no power for acceleration and speed will gradually decline. If the car is turned off and then back on, the problem clears...for a few minutes. I have an Innova 3100e code reader indicating that there are no codes, which is consistent with the lack of 'check engine' light.

According to the previous owner, the fuel pump was replaced, twice, at a Ford Dealer with no change to this problem.

I have:

replaced the battery and cleaned the cables (battery was six years old)
replaced air cleaner, plugs, wires, and coil pack
cleaned MAF
checked fuel pressure at fuel rail and the Fuel Pressure Sensor - reads 35 PSI and 40 if I remove the vacuum line.
replaced CCRM module with one out of an '01
tested TPS sensor - it reads 1v at idle and 4.6 v WOT
had alternator tested at local parts store, they said it passed - I thought it might be bad because I noticed the headlights dim a little when coming to a stop - also only reading 13.5 volts when the car is running (any other car I've had reads 14.4 ish)

Although the check engine light came on when I had the MAF disconnected, I thing maybe the PCM is having an issue.

Any ideas regarding this issue will be appreciated.

 

[TGIVinnie]

Saw a reply in my email, but I don't see it here asking about codes.

That's one of the frustrations, it's not throwing any.

 

[squeak93]

Does the tps voltage move through the range smoothly when you're opening and closing? I ask because we had a mod motor car show similar issues. When we tested the tps initially it showed proper open and closing voltage. What it was doing was loosing the mid range voltage and "jumping" around in the range causing some major drive ability issues. We replaced it and the car ran excellent after.

 

[wmburns]

At 13.5 volts the alternator voltage is too low. The low end of the range is 13.7 volts.

I recently rebuilt an alternator because the "standard" voltage was 13.8. While the alternator passed all tests I found that battery life was short. Replacing the battery every 9-18 months. After rebuild the alternator voltage was always above 14 volts.

Here's some information about how to perform voltage drop tests to determine if something external to the alternator is causing the voltage to be too low.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Might consider performing an engine compression test just to rule out a base motor problem. This motor's vacuum seems a little on the low side to me.

If there's air trapped inside the motor the ECT will read low. This creates a situation where the motor could be running hot but the dash gauge reads low.

V6 coolant refilling procedure:
http://www.allfordmustangs.com/foru...stang-irregular-rpms-showing-hot.html#2604913

 

[TGIVinnie]

Yes, the TPS tests fine, but of course that's in run and not moving. I actually left the paper clip I used on the connector to back probe it in place and when the car started acting up I left it running and checked the TPS voltage. Moved the throttle some, the engine chugged and sputtered at me and the TPS voltage read about 2.5 v.

When the car gets into its 'mode' it even idles bad. I let it chug and sputter in the driveway today, until it finally stalled. I was hoping it would toss a code at me.

And as always, if I turn it off and then back on, it runs normal...for a few minutes.

I ordered an OBD to wifi adapter that will hopefully allow me to collect some live data on it. That won't arrive until Tuesday though.

 

[squeak93]

I missed the voltage earlier, I agree it's too low. Start there and I bet a lot of your issues solve themselves.

 

[TGIVinnie]

Coolant levels look good.

Alternator is questionable. When I first start the car, I get 14.3 volts, as the car warms up (and the alternator gets hotter than it seems like it should) it drops down to 13.45 with the AC and headlights on. If I turn on the rear window defrost, the voltage will drop to 13.0.

Did the voltage drop test, too. Negative to Alt case was .02 v Positive to Alt + was .06 v

 

[TGIVinnie]

Update: I bought a replacement PCM: it didn’t work when I installed it (needed to take it to a shop and have it programmed for the security module) so I put the old PCM back so I could drive to a shop....haven’t had problems since (still running the original PCM) and I’ve put about 7000 miles on it

Asking around locally, it’s fairly common for these cars to have connector issues. In the process of installing the new PCM and then reinstalling the original I probably corrected a poor connection issue.

Hope this information helps the next person