I recently did a complete rebuild on the 302 in my 91. New heads, new cam, crank turned 10/10, cylinders .030" over. I dropped the engine in, got everything buttoned up and fired it up. It had a rough idle, which was to be expected, but it never went away. With the rough idle, it is also extremely doggy, and running a little rich. When I put it in gear or kick the ac compressor on, it stalls a little.
Timing is spot on, all vacuum lines have been checked (as well as every other possible vacuum trouble spot), fuel system has been checked, had it hooked up to computer and its not throwing any codes. I tried everything I possibly knew to no avail. A week ago Monday, I took it to a ford dealership. After a week of them prodding on it, they can't find the cause of the problem either.
Any help would be greatly appreciated!
On a side note; At some point it has had a swap with a mustang, or an owner down the line has just put a timing cover from a 5.0 mustang on it. The only way I could find a water pump with the right plate was to get a 91 mustang 5.0 water pump.
Thanks!

91 Bronco 302 4x4 auto

 

"No codes" is a questionable response since there are NEVER "no codes". Even if the system is functioning perfectly there is at least one Code generated with every KOEO and KOER test that is run.

But before we assume there is something amiss that can be found via diagnostics testing, answer a question... Was the IAC (Idle Air Control) valve and solenoid cleaned or replaced when the engine was replaced? If the IAC has not been cleaned lately, remove it from the throttle body and inspect it for coking and crud in the valve body. Clean as necessary and reinstall. There is a procedure for checking the IAC for proper operation however, since you say there are no fault codes being generated it is unlikely the problem lies there.

There are two sensors in the system that can be "off" and still not trigger any additional fault codes. The ACT and ECT sensors are thermistors and as such the ECM "expects" to see a certain resistance range from them. The problem with this situation is the fact that if either sensor "sticks" at a resistance value "within expected parameters" the system will assume there are no issues when in fact, the reading being sent to the ECM is inaccurate for the temperature of the air coming into the intake (ACT) or the temperature of the engine coolant (ECT).

With this condition in an ECT sensor symptoms include: hard start when cold, frequent stalling or miss in cold weather and during the first 10-20 minutes of operation (depending upon ambient temperatures), premature spark plug failure (stuck hot condition) OR hard hot restarts, rich running even after engine warm up, poor fuel mileage, soot-blackened spark plugs (stuck cold condition) Either requires replacement of the ECT sensor and will not necessarily generated a fault Code.

ACT sensor problems typically manifest as rough, erratic idle, poor throttle response, stalling, and/or running rich (depending upon which side of the programmed thermal threshold the thermistor "sticks".

There are ways to determine whether one of these sensors has indeed "stuck" however, they require removing the sensors anyway so replacement is almost a forgone conclusion. (If you are going to go to the trouble to pull a suspected faulty sensor, why not just replace it?)

 

Thanks for the reply Greystreak!
Sorry, to mop up my confusion, I did get the 111 "nothing is wrong, dummy" code.
I replaced both temp sensors when I did the rebuild. The IAC was replaced a couple hundred miles prior.

 

Ah ok, then. Then whatever is causing the issue is not enough of a detriment to make the ECM "think" something is wrong. You might take resistance readings on both the ECT and ACT with the engine cold just to verify that the initial resistance value is indeed where it should be given the ambient temperature. I have posted the resistance chart of both of these sensors a dozen times or more but if you decide you want to check them and can't find it, just let me know.

 

i have also read at times that the TPS can cause this issue and not always trip a fault code.
was this motor put back together with stock 91 parts such as the cam & heads or are those parts been upgraded?

 

Got it narrowed down to a dead #3 cylinder. Pulled the plug wires one by one and #3 caused no change in idle. Don't make fun of me too bad for this not being one of my first steps. Swapped plugs with a different hole and it's still #3. So I know the plug is good. There is spark when I ground the wire to the exhaust manifold. Hoping its just a weak spark caused by a bad wire, cap, or distributor, and not a dead brand new cam.

 

It could be a crossfire caused by other of the cables. Check that the cable that fires the cylinder that's ahead of #3 isn't making contact with its cable.

 

I'll look all the other wires over real good, Encho. I don't think that would be the case though. 3 fires between 6 and 7, both of which are on the opposite side of the engine. If it is doing that it would have to be doing it at the cap. My luck, either the cam is flat at 3 or the tappets didn't pump up.

 

was the 15min 2000 rpm cam break in procedure followed right after motor got fired up? get a compression gauge on that cylinder also

 

Yeah I did the break in for the cam. Did a compression test on Saturday.
1: 175psi
2: 175psi
3: 180psi (this is the dead hole)
4: 165psi
5: 150psi
6: 165psi
7: 175psi
8: 175psi

 

I'm going to check to make sure I didn't bend any push rods today (and hope I get lucky there) before I go through the trouble of pulling the cam.

 

for a fresh motor build those numbers are not good.

you should pull the valve cover off and see if the vale/push rod moves the rocker.

 

I agree ^ those numbers aren't that good