Enigne hunting and stalling

[mrsolo2000]

Before I continue to throw parts at the problem and chase ghosts....I could use some insight here. I have a 1990 B2 4WD that I swapped in a 4.0l OHV engine. The block came from a 1996 Explorer but the intake, fuel system, and engine control (i.e., EDIS, ECU, etc.) are off a 1991 Explorer. Engine runs great but does "hunt" a little bit in RPMs when you take your foot off the gas after accelerating. The hunting is about 50-100 RPMs but settles out at a steady idle at around 1000-1200 RPMs after about 2-3 seconds (this is while the car is moving down the road). Sometimes, at low speeds pulling into parking lots or coming to a stop, the engine will stall altogether. I've checked for vacuum leaks (I don't think I have any). I replaced the IAC. There's a new fuel pump in the car. When the engine is running, there's 30psi in the fuel system. According to the Ford tech manual, 30psi is the bottom end of acceptable fuel pressure (I'd like to see 35psi or more....). I also replaced the fuel pressure regulator too. I have toyed with the idle set screw at the throttle body. When the engine is warmed up, I have the base idle set around 900 RPM. I think that's within range but I am not 100% sure.

Any Ideas?

One thing to consider that may be an issue...I am using the injectors from the 1996 Explorer and not the 1991 Explorer. I looked up the two injector types and they have identical flow rates. However, the injectors from the 1996 year have a different spray pattern. Does the spray pattern make a difference? I do have the injectors from the 1991 but wanted to get opinions before I take the upper intake off to swap injectors.

 

[Shran]

Does it have an automatic transmission as your profile says? And what was your '91 Explorer donor, auto or manual? I used an auto ECM with my V8 swap and I have a manual, it will hunt for idle at stop lights occasionally... had a similar problem with a 4.0 swap years ago but it wasn't totally stalling. '91 was pretty primative so at least you don't have cam sensors and EGR to mess with.

I don't think the injectors will make a difference. I have heard the newer ones are an upgrade but I don't know if it's really a noticeable one.

I think I would start by checking your throttle position sensor voltage and see if there are any dead spots. Should be around 0.75-1v with the throttle closed and up to 5v at WOT. Adjusting your throttle stop screw will change the idle voltage so you have two variables there that affect each other.

 

[2011Supercab]

I replaced the IAC.-----I have toyed with the idle set screw at the throttle body. When the engine is warmed up, I have the base idle set around 900 RPM. I think that's within range but I am not 100% sure.

IAC controls engine idle speed.
With engine at operating temp unplug your IAC, idle should drop to 500 RPM or lower, maybe even stalling engine.

But because you messed with the "idle set screw" it probably won't.

With IAC unplugged adjust the "idle set screw" until RPM are about 500, then plug IAC back in, rpm should go up.

Also what brand IAC did you use, Ford only likes Hitachi or Motorcraft.

 

[mrsolo2000]

Does it have an automatic transmission as your profile says? And what was your '91 Explorer donor, auto or manual? I used an auto ECM with my V8 swap and I have a manual, it will hunt for idle at stop lights occasionally... had a similar problem with a 4.0 swap years ago but it wasn't totally stalling. '91 was pretty primative so at least you don't have cam sensors and EGR to mess with.

I don't think the injectors will make a difference. I have heard the newer ones are an upgrade but I don't know if it's really a noticeable one.

I think I would start by checking your throttle position sensor voltage and see if there are any dead spots. Should be around 0.75-1v with the throttle closed and up to 5v at WOT. Adjusting your throttle stop screw will change the idle voltage so you have two variables there that affect each other.

Thanks, good stuff! The 1991 stuff came out of a donor vehicle in the junk yard and it didn't have the transmission in it and I cannot recall if the interior was setup for a manual or auto. I think the ECU numbers are a california MT ECU but I could be wrong. I'll check the throttle sensor, thanks again!

 

[mrsolo2000]

IAC controls engine idle speed.
With engine at operating temp unplug your IAC, idle should drop to 500 RPM or lower, maybe even stalling engine.

But because you messed with the "idle set screw" it probably won't.

With IAC unplugged adjust the "idle set screw" until RPM are about 500, then plug IAC back in, rpm should go up.

Also what brand IAC did you use, Ford only likes Hitachi or Motorcraft.

I may have an IAC problem....I did try and troubleshoot by unplugging it while the engine was running and there wasn't much of a change. I'll lookup a motorcraft IAC.

 

[Shran]

If the ECM is for a manual it might be looking for input from the speed sensor and clutch switch to keep the idle up when slowing to a stop and if that’s missing, it could somewhat contribute to the stalling… if not the hunting as well but I feel like the IAC or TPS are in play here too.

 

[SenorNoob]

Definitely get that base idle turned down. 900 is too high even for the PCM control. Should be roughly 750 warm with the computer.

The injectors "shouldn't" be a problem. I ran 10 years on the newer style injectors with my 92 PCM.

If you're using an automatic PCM, you should have codes for that. IDK about how the California part would factor in.

 

[mrsolo2000]

I finally got around to checking the TPS voltages and I am left scratching my head. I placed the negative lead of the multimeter on the BK/W pin (which is the ground according to the vacuum and electrical troubleshooting diagram) and put the positive lead on what I believe is the wiper pin. With the KOEO, I measured -4.2V at idle and -0.56V at WOT. That's right, negative values. Is that right? Am I having some brain flatulence?

 

[SenorNoob]

The voltage sounds right except completely inverted...

My diagram shows Brown/White is the 5v reference signal. Gray/Red as ground. Gray/White as signal to PCM.

 

[mrsolo2000]

I replaced the TPS just because. The old TPS was on the intake when I pulled it out of the donor car in the junkyard so I figured I'd go ahead an put a new one on. I did some more driving and discovered some more quirks to my hunting issue. At park or at speeds lower than 25-30MPH, there are no issues. I can rev the engine and take my foot off the throttle and the RPMs aren't hunting as it goes back to idle. At speeds above 35MPH, I can go WOT and take my foot off the gas and the RPMs are smooth all the way down to idle. It seems to be the spot between 25-35MPH where I am having issues with the engine/ECU stabilizing at idle after taking my foot off the gas. I didn't catch what gear the A4LD transmission was in but I suspect it was in 4th. Isn't there a torque convertor lock that occurs somewhere in that range? For reference, the A4LD is from the 1990 Bronco and the ECU, ignition system, and intake is off the 1991 Explorer. I am about 100% sure the donor Explorer was an auto transmission (recall that it was missing out of the donor vehicle in the junkyard). I'd have to check the part number of the ECU to verify if it's an AT or MT part number, just haven't had the time to get to it yet.