I just bought a 1990 F150 that I just rebuilt. Before rebuilding the engine idle would surge between 400 rpms to 1400 rpms. While rebuilding the engine I discovered that it was a stock roller motor with a .030 overbore. I now have the engine re-installed and the surging idle is still there. I found the TPS was bad and replaced it with a new one and set the volts to 1 volt and tested to ensure the new one works and it checked out good. Started the truck and the idle is still surging. I have also replaced the knock sensor, air charge sensor, temperature sensor and the EGR valve. Still the idle is surging. I have checked all of the vacuum lines for leaks and zero have been detected. Any help with troubleshooting will be greatly appreciated. I have been reading past post on this same issue from others and have not had any luck. One other question I have is will a 302 roller motor work with the stock 1990 PCM? I know the firing orders are different between the two and I'm wondering if this could be the cause to my issue. What do ya'll think? Thanks in advance with any help! I'm at my wits end with this thing.

 

From what I've read, this is usually an IAC issue.

 

Sorry left that one off. I also replaced the IAC as well and still no luck! Thanks for the reply!

 

Ok, I should be able to help on this one..........

1) The 1990 F150 did NOT come with a stock roller cam.

2) Check the vacuum hose that runs to the MAP sensor to ensure it is not blocked. Spray some carb spray down the hose if necessary.

3) Seeing that you do not have a stock camshaft, you will likely need an IAC bypass plate because the camshaft you have (assuming it's not a 92-93 roller camshaft) is requiring more airflow at idle.

4) Make sure your PCV valve is a Motorcraft only unit.

Let me know how this works out and if it is still surging, we'll go a bit further.

 

I re-read your post. Firing order doesn't have anything to do with it and the 1990 ECM will run with either firing order.

 

The roller cam won't affect the ECM in any way. There are 2 versions (firing order). There is the old school firing order roller cam which you'll find in the 92 - 93 F150's. Then there is the HO firing order which they started putting into the f150's in 94 -5-6.
Same motor basically just different valve order so different firing order.
They actually started putting the roller cam engines in F150's in 87 - they just put a flat tappet cam in until 92 when they started putting in the roller cams. Then in 94 they put in the HO roller cam (also when they changed from speed density to MAF management).
Upshot is that a 90 motor will accept a roller cam. Speed density (yours) will work ok with either firing order.
You will need a different distributor gear to go with a roller cam.

 

TPS should also be motorcraft part, other brands just seem to cause headaches-such as surging idle.

 

My 1990 5.0 also had the dreaded surging idle. After replacing literally everything on the engine and replacing my tps,IAC, and map sensor with a quality motorcraft part and installing the IAC spacer that qdeezie mentioned it finally idles perfectly. I too have the roller cam '94 up 5.0