I got gremlins

depending on how long this is been happening,.. i had one night that happen then never again i got dumbfounded on what it might be or have been and i narrowed it down to bad gas.. but after that night i noticed the same thing your discribing happen to mine and it was just a bad tank of gas.. it had to for my problem!

The cylinder balance test (in my experience) is much more sensitive than even our butts, so that's weird.

I know you have fresh plug wires but have you checked for spark leak at night (misted water works well to make Peter Pan dust)? It's all a guess but that was my first thought. Relative proximity, even on new wires, can matter.

That's my best guess for now, though it's of no real help............

Good luck.

As suggested by HISSIN50...

Cylinder balance test:
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at Walmart.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $33.

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

i would think that if it were the injectors it would be more spurratic and do it most all over the rpm range.


so my guess is possibly the computer or chip.

also hissin or jrichter may know but i think there is something that helps the car run at lower rpms but does nothing on the higher rpms.

Could also be a vacuum leak. That would have less effect as the throttle was opened.

Nachoman427 said:

Could also be a vacuum leak. That would have less effect as the throttle was opened.

Click to expand...

i thought the same but chose to post another thought. check the vac lines and psi test them

I've had injectors that were partially clogged/varnished a bit after a period of non-running. The issue was exacerbated at cold idle, low RPM. Once the car warmed up, it ran much better, and after a couple hundred miles, the injectors were doing much better overall.

Not that this is your issue, but rather just my experience with poopy injectors. Did this issue start after any recent work or mods?

If you havent, ensure the EGR pintle is not propped a hair open by a piece of carbon.

I'm sure JRichker and others will have some better advice.

Good luck.

id run it untill its empty then fill up with a higher octane, i always use 94 oct.. its real cheap right now try that... if you dont already use it.

what about your fuel pressure ? do you have a guage ? or have you tested it ?

im sorry man, i really dont know whats goin on with your car.. i just thought it might have been the same prob with the bad gas that i had, becuz the same thing was happening to me.. my pressure reads 20 and jump higher when i give it gas..

90mustang_GT5.0 said:

im sorry man, i really dont know whats goin on with your car.. i just thought it might have been the same prob with the bad gas that i had, becuz the same thing was happening to me.. my pressure reads 20 and jump higher when i give it gas..

Click to expand...

As you know, that's not right.

i have an exterior fuel pressure guage and it reads 20/ 25 at idle, and when i get on it, it jumps to about 38/40

Your fuel pressure should never go down. If it does you have a restriction in the fuel supply line, a bad regulator, or a bad pump.

Sounds like the regulator is iffy. The pressure should increase as soon as the vacuum drops. There shouldn't be any lag time.