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RPM Issues
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Posted by: Mustang Sally

Just recently bought a 91 GT. And she can't decide whether she wants to run or not. Started having RPM problems once in a while. Under normal driving conditions RPM's would jump up and down (sometimes will back-fire) and always stall out. Have changed out Mass Air Flow module, Ignition Swith Module, blown out the throttle body, ran fuel injector cleaner, changed out alternator\battery while this was going on. Got a diagnosis from Ford came back as spark\spark wires. Put Double Platinum plugs\better plug wires in. Already had MSD distribiter. Everytime I think I've gotten it, she starts acting up again!!! If I don't fix it soon my husband's going to make me sell it!!! Please someone help!! It has 79,000 original miles. Already checked timing, it's o.k.

Posted by: Mustang5L5

Well while i don't think it's the cause of your problem, the platnium plugs don't help. 5.0's tend to run poorly on platniums. The 5.0s run better on copper plugs, preferably Autolite 25's gapped at 0.054". Copper plugs are a better conductor and make more power despite what every parts store guy will say

Here's a stupid question, but one i ask because even I once messed this up and spent 2 weeks chasing it...but is the firing order correct? It's 1-3-7-2-6-5-4-8

Did Ford hook it up to the computer and pull codes? WHat exactly showed up?

If not, head to Autozone and have them run the codes. They usuallyt offer the service for free. You can buy a code reader for $30 or you can do it for free by reading this website. All you need is a paper clip to trigger the Check engine light to flash the codes to you.
http://www.corral.net/tech/maintenance/eecivtest.html

Even if the CE light isn't on, there still could be codes stored that would give us a better idea on what is going wrong. My guess is it's EGR related. Take the EGR valve off the back of the intake and clean it out well. Make sure the plunger can move. A lot of times they gunk up with soot and cause part throttle problems and backfiring.

Another fix is to locate the 10-pin connectors behind the intake. These look like salt-and-pepper shakers. There should be two of them..one white and one black. Sometimes there are 4 as Ford installed a extension harness in '89. If the extension harness is present, remove it as it causes problems. Apply fresh dielectric grease and make sure the connections are nice and tight. This is the conections for the fuel injector harness. It sometimes works itself loose and causes problems.

Posted by: Mustang Sally

Changed out TPS last night, ran great thought that was it. Ran it today still acting up. Gonna go with what you suggested. Thanks so much for the advice. By the way, GREAT looking Mustang!!!! Talk to you soon. Hopefully we'll get it fixed!!