HELP: diagnose engine prob '85 Mustang

Aug 07, 2004 4 Replies

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J.B. Bobbitt 21 years ago

1985 Mustang LX
3.8 L V-6 CFI
81,000 miles (total)

Problem: The car starts and runs well, for the most part. But at steady cruising speed (mostly 65-75 mph, less often at 35-40 mph), there's a intermittant hesitation and loss of power for 2-3 seconds. It feels like the car is hit with a big blast of wind. Usually, it picks up again and runs fine. Rarely, the engine stalls out (kind of spooky at 70 mph on the freeway), but restarts after 2-3 minutes of sitting. Sometimes, on hot days, it runs rough and stalls in stop and go traffic. The problem seems to happen most often on hot days (95+).

So far, I've done a major tune up including fuel filter, cap & rotor, plug wires, &tc., and I've replaced the TFI module.

Any ideas or advice?

Thanks a heap,

-jbb

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Backyard Mechanic 21 years ago

J.B. Bobbitt opined in news:RJ7Rc.4174$ snipped-for-privacy@newssvr29.news.prodigy.com:

Sounds like the fuel pump. Dunno if it differs from the regular hi pressure one.

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69GT4spdtrac-loc 21 years ago

I would get the module checked on the distributer. Ford is famous for these to go. It usually acts up when hot or warm.

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JD 21 years ago

I concur. A flow/pressure check may or may not pinpoint the true cause as it may also be a pump relay or rollover switch problem. And of course, it probably cannot be replicated while the car is up on a lift in the shop. If the pump is well aged, I'd start by changing it out, and if the problem doesn't subside, have the electrical checked out.

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J.B. Bobbitt 21 years ago

I replaced the TFI module earlier this morning and took it out on the freeway (ambient temp ~80 deg). The problem was there but not too bad. I took it out again mid-day today (ambient temp ~96+), and the problem was A LOT worse: rough running at low RPM and low speed as well as the hesitation at high RPM, stalling at idle, &tc. The problem definitely seems worse at hight ambient temperature.

The shop manual I have says there's a low pressure, in-tank fuel pump as well as a high-pressure, in-line pump. I'll start checking those, I guess.

Thanks for the help, guys.

-jbb

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