1990 Mustang Bucking Problem

Um..Hello would be nice. What on earth makes you think it is a fuel pressure regulator? And who goes 30 in 3rd gear? LOL. It's a spark miss. You are under load and the spark jumps. Cracked plug, bad wire or cap. That'll be $50.00.

I would look toward the wires truthfully. Run it at night with the hood up and lights out. Look for sparks. If ya wanna be crafty, use a spray bottle with a little water. Don't drench it but spray the wires lightly and watch for a spark. Second guess would be cracked plug.

I'm not gonna tell you to spend money without seeing it first hand. Good functioning stock replacements would cure the problem and MSD or Taylor's are nice but expensive. Yes they are the way to go. No I'm not gonna guarantee a fix without looking at it.

Surge at part throttle cruising, backfire on accel, high idle, rough or uneasy idle, lots of things. And if its vacuum to a sensor that creates more symptoms. 1500 rpm is low to be in 3rd gear so the engine is under load. Just like 5th gear at 45 or 50. When you load the engine like that it tends to make spark jump from a bad wire or plug. Thats my call. It could be cured as simple as putting tune up grease in the plug boots. But they probably need replaced.

Does nobody check codes anymore? Just in case? Must be the subframe connectors.

Just haven't read this until now...

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way 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. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.





If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.

What cam do you have? If I try to go a steady speed around 25 to 30 MPH in 3rd, my FMS B cam likes to buck. Coasting down or acceleration are fine.

Speaking of codes...a dirty MAF could do this.

Check your timing and O2 sensors

My MAF is new and it bucked (jerked like a work truck?) with the stock one. But I do need to check timing to see if it is still advanced to race specs.
Would clocking the MAF be of any help here?

Did Mr. R get his codes yet and what cam is in it figured out? I bet with the "rev by itself" there is something that needs attention, even if it does not fix the buck at steady speeds. It is am manual transmission, right?

could be pip sensor in dizzy......reving by itself could be another problem vac leak or dirty mass air..........codes?

What's done to the engine?
When I had a miss-matched MAF to my injector size it would cause a surge and I'm pretty sure that would have cause a buck if the engine had a load on it like what you were saying.
But check codes first before you go throwing parts and money at it.

Luciano Rodriguez said:

The egr has been removed and is blocked off but the plate doesn't seem to seal would this cause a major problem?

Click to expand...

It would cause a vacuum leak.