90/91 4.0 OHV issues in my Choptop

[lil_Blue_Ford]

So, I still have some wiring to sort out that may or may not be influencing anything from my swap since this originally had a 2.9.

I’ve been trying to sort out why it’s not running it’s best and why it’s running rich. I’ve managed to get it better than it was but I recently pulled codes and here we are.

I’ll start with what’s been replaced…

ECU - this was mainly done because I’ve been running an auto computer in a manual but also my capacitors popped and burned a couple resistors on the board. Replacement is a reman for a 91 Ranger 4.0 manual.

O2 sensor - I couldn’t remember when it was last changed. Got a Bosch from the local AutoZone.

TPS - apparently I somehow used a 2.9 TPS and it somehow worked despite turning the wrong way? Either way, it has an aftermarket 4.0 TPS now.

IAC - threw it on just for giggles.

BMAP - yeah, apparently the early 4.0 wiring harness will plug into a MAP sensor but it’s not supposed to. Deleted the MAP sensor in exchange for a barometric sensor. This did give a boost to fuel economy and was my most recent fix.

Now… I’m back to getting a high idle. It also has a hot start issue if I don’t let the E-fan run for a couple minutes when I shut it down, when I try to restart it has extended cranking and sometimes takes a couple tries to fire. Have not yet done a smoke test for vacuum leaks but it’s on the list. Fuel economy isn’t quite what I think it should be either. Pulled codes and I’m not sure what to make of it. Tried to pop the vacuum line off the FPR but it was stuck on there good and I haven’t got back to checking because it’s been on and off rain and cold the past two days.

I got a code 67 on the O side which the book says is a clutch/neutral safety or AC wiring issue which I suspect is in the wiring issues I’m trying to sort out with the connections made between the 89 Bronco II harness and the 90/91 Ranger harness. Having a little trouble trying to sort that out despite having EVTMs for 89, 90, and 91 but I’m still working on it.

The other code was a code 14 on the C side which the book says is an ignition PIP fault. Initial google search tried to tell me it’s a bad TFI or distributor pickup, but this has neither, just the ignition module on the core support. One other thing I found suggested a problem with the crank sensor. Not entirely sure what to make of it.

Hoping someone has some suggestions of what direction to head or some input on the wiring. I’ll keep studying the manuals, for whatever reason I’m finding wire colors that don’t match the books and connections in the manual that don’t specify what connector they go through. Of course, none of the plugs I need to work on have a pinout page so I’m stuck searching through the book.

 

[Shran]

From what I recall some early 4.0's had both a MAP sensor and a MAF sensor but all 4.0's had the latter. I'm not really following what you did there. A lot of trucks from this era are confusing as far as the MAP sensor goes - some need engine vacuum and some do not and are just open to the atmosphere. I'm not sure what to suggest there - it would depend on what the ECM is calibrated for.

High idle and hard hot starts could indicate to me that you are dealing with vacuum leaks or a leaking fuel pressure regulator or both but your code 14 might be in play there as well. A little bit of Googling I did suggested that it might be caused by a misfire or could be an ignition module or crank sensor problem on a 4.0. I think you will need an oscilloscope to test either of those. Check your wiring and swap with spares if you have them.

I don't think I would worry about the code 67. IIRC you can get that depending on AC on/off condition and whether or not the clutch pedal gets pushed in during the test.