I have a 1992 Ford Ranger STX 4x4 with the 4.0L OHV and i have been having issues with a high idle. At the moment my truck idles about 1250rpm and its only given me running lean codes. I've replaced the fuel pump, fuel filter, injectors, IAC sensor, Air temp Sensor, throttle position, Oil pressure sensor, O2 sensor, better flow headers, new cats, and replaced the whole engine (due to my oil filter falling off while driving and murdered the engine). I've been chasing this problem for a long time now and haven't gotten anywhere. I've gotten tips to do a full clean and coat on my lower and upper intake manifold. I don't think it would change anything if i were to do a full clean and coat since my O2 sensor still inst switching and says its running lean. I've gone through and checked the wiring harness to the O2 sensor and its reading the full volts that it should. I'm really at a loss and in need of some experience with this kind of problem.

Note I have gone through all gaskets related to exhaust and have no leaks.

I also have a oily clutch issue, but I have no leaks on the top end of the engine that would be causing that issue, just super burnt out and I hope I'm not the only one who dealt with this issue.

Constant Codes are 172 and 411

 

Pull out the computer and have a look at circuit board, just to take that off the table, there are 3 blue capacitors that can fail and leak after 20+ years

Idle should be high at Cold Start, then slowly drop as engine/coolant warns up

Generally speaking gasoline engines use air as the RPM control, the point of the throttle plate
And the point of using an IAC(idle air control) valve

After engine is warmed up and idling, unplug the 2 wire connector on IAC Valve
RPMs should drop to 500 or so, engine barely running, or engine may even stall, either is good means no Vacuum leaks and IAC valve was working

If engine RPMs do not drop then most likely there is a vacuum leak and that also cause Lean Codes, but this should also set an RPM HIGH Code
Lean codes DO NOT mean the engine is actually running lean, lean codes mean the computer is having to add more fuel than it calculated, if there is a vacuum leak then "unreported air" is coming into the engine so computer has to add more fuel to compensate for it

With IAC Valve unplugged and engine still idling, high spray soapy water around the intake, safer than flammable stuff, engine will stumble if it sucks in soapy water and worst case is you get a cleaner engine, lol

You can unplug PCV hose and plug port with finger, same for power brake booster,these can be the larger vacuum leaks

 

I went ahead and checked out the computer and everything still looks like its brand new. There were only 2 blue cylinder capacitors on my ECM, but all other pieces looked in working order and no scaring.
I also took off the upper manifold and replaced the gasket and cleaned it up the best I can. I took out the sensors and checked for scaring. The only sensor that was scared was the air temp sensor, but it threw no codes.
After I put it all back together, I started it up and it and it running very poorly. She idle correctly around 650-700 and never goes above 1k when coming to stops. The new problem is that she is has really bad hesitation and is producing a misfire. I've rechecked my firing order twice and it all lines up.
I do have the vapor canister vacuum line disconnected with a vacuum stopper to see if that is where my leak is coming from. My plan is to let her cool down, and do a reset on the computer to see if its just the truck not switching to the right mix of fuel/air.
Yesterday I was driving her and she produced a loud whistle noise when pressing the accelerator, following that she felt like she was bogged down and has much less power to give. Am I wrong to think that there could possibly be a timing issue at play? The new block was preassembled with heads and was timed from the factory, could it be possible when driving that the chain slipped causing my loss in power, the whistle noise, and the rough Idle?

New code list: Only 411

 

No on timing chain on a 4.0l OHV, they are bullet proof

Does read like a spark plug/coil issue
3 4
2 6
1 5
front

5, 6, 4 side is easy to mis-wire, at the coil or the spark plug end

Vapor canister should be hooked to a Ported Vacuum port, on under side of throttle body but in front of the throttle plate, so not "vacuum" at idle, has vacuum at higher RPMs, but just a few inches not like regular vacuum

411 (R) Idle speed system not controlling idle properly (generally idle too high)
This may be an old code when idle was too high
Clear it and see if it comes back

 

My firing order matches that pattern. I'm sure the Vapor canister vacuum line can still cause a leak at idle. I've taken it off before and my idle went all choppy. I see what you mean with the vacuum control, is there a way to test that because I feel like it is stuck open. I plan on letting her cool down and clearing the codes. The one thing that confuses me is that she is only throwing the 411 code and not the previous 172 code. I'll have more answers tomorrow when I attempt to fix her again. Thank you very much for helping me out. Hopefully i can get her straightened out so she can be back on the road enjoying life.

 

Well to end on a bad note. I have found out the reason behind my Whistling noise during acceleration and why she's down on power. She sadly has a blown headgasket. Hopefully I can have it warrantied out and get here a fresh engine again. Thanks again for all the help! I'll get her back on the road someday for sure.