2.5L ('98-'01) Occasional low idle and other issues

[holyford86]

Truck is a 98 with a 2.5 and manual trans. I've been having an occasional issue with low idle (350rpm) and sometimes is accompanied by a stall, seems most prevalent when electrical loads are high (headlights and blower on) I can't force it to do it, it seems to just do it when it wants to, there are no codes and idle is maintained 99 percent of the time. Truck has a hair over 100k miles, tune up stuff was done when I bought it 50k miles ago and used all Motorcraft parts. Voltage stays consistent and truck has 130 amp alternator on it because it's what I had when the bearings failed on the old unit. I don't feel the IAC is bad because swapping to a known good unit didn't change a thing.
Sometimes I get what feels like a short duration loss of spark but I'm not sure if it is related to the idle issue or not, seems more prevalent when it's cooler and damp. Is this a sign of a failing crank position sensor in these? I've always worked under an assumption that they either work or they don't and there is no in between. I can hook a scope up to it and see if the crank sensor drops out or watch live data to see, but I was curious if either of these are a pattern failure on these before I go to the trouble.

 

[RonD]

No, I wouldn't think crank sensor either, they do tend to be reliable and either don't work at all or work, not intermittent

The computer maintains idle +/-4rpm so should react pretty fast at dropping RPM like that, does the engine "race" at the times it recovers from low RPM, i.e. computer has opened IAC Valve more to raise idle up as it drops

After engine is warmed up and idling normally, unplug the 2 wire connector on IAC Valve
RPMs should drop, but only to 500-600rpm, barely running but running

If engine stalls you can adjust the Anti-diesel screw, looks like an idle screw, on the throttle linkage
Leave IAC valve unplugged and turn screw in to open throttle plate a bit more to set 500-600rpm idle

This is NOT your issue but can prevent the stalling when issue happens

Could be dirty sticking PCV Valve, its a "controlled" vacuum leak, so part of the idle air allowance
Sticking dirty EGR valve, if you have one, leaking exhaust into intake at idle

Dirty fuel filter, long shot but not a no shot, lol

1998-2000 was when Ford changed Rangers to 60psi "returnless" fuel system
True "returnless" system
Inside the gas tank on the fuel pumps OUT hose is a Fuel Pressure Regulator(FPR) set for 60psi
This setup could/did cause wide fluctuations in fuel pressure at the engine end

In 2001 they switched to a short return system, 3rd port on fuel filter, not sure if 2.5l had this in 2001, but doesn't effect your 1998
In any case the fuel pump OUT went directly to fuel filter and out to engine
The 3rd port on filter went back to gas tank and the FPR
Fuel pressure at the engine end was way more stable with this setup
Extreme long shot, lol

 

[holyford86]

No, I wouldn't think crank sensor either, they do tend to be reliable and either don't work at all or work, not intermittent

Thank you for confirming my suspicion

The computer maintains idle +/-4rpm so should react pretty fast at dropping RPM like that, does the engine "race" at the times it recovers from low RPM, i.e. computer has opened IAC Valve more to raise idle up as it drops

The one occasion where it did this when I wasn't inside driving it, it went to die and I could hear the IAC open up but it didn't really overshoot by much, it maybe made it up to about 1100 rpm for a second then back down to around 850 or so and stayed there.

After engine is warmed up and idling normally, unplug the 2 wire connector on IAC Valve
RPMs should drop, but only to 500-600rpm, barely running but running

If engine stalls you can adjust the Anti-diesel screw, looks like an idle screw, on the throttle linkage
Leave IAC valve unplugged and turn screw in to open throttle plate a bit more to set 500-600rpm idle

This is NOT your issue but can prevent the stalling when issue happens

As a last resort I may take this route if needed.

Could be dirty sticking PCV Valve, its a "controlled" vacuum leak, so part of the idle air allowance
Sticking dirty EGR valve, if you have one, leaking exhaust into intake at idle

Egr isn't something I had thought about as an issue and it seems to happen mostly when coming to a stop after mid throttle and somewhat low load driving, which is when EGR would be in operation if I'm not mistaken. This issue became somewhat worse after replacing the rotted out egr tube, I'm just tempted to pull the vacuum line off the valve and go for a short drive to see if it changes. I had changed the dpfe sensor a few years ago because the original pot metal unit was clogged with corrosion. I'm not a fan of the parts cannon but would consider tossing an EVR and or valve on the truck (but I'd prefer to test and see first)

Dirty fuel filter, long shot but not a no shot, lol

Its about due, it's on the list for my spring maintenance, haha. Has plenty of power and isn't a large worry though, the filter has about 28k on it. This was an issue when the filter was brand new also, it's just gotten slightly worse lately.

1998-2000 was when Ford changed Rangers to 60psi "returnless" fuel system
True "returnless" system
Inside the gas tank on the fuel pumps OUT hose is a Fuel Pressure Regulator(FPR) set for 60psi
This setup could/did cause wide fluctuations in fuel pressure at the engine end

In 2001 they switched to a short return system, 3rd port on fuel filter, not sure if 2.5l had this in 2001, but doesn't effect your 1998
In any case the fuel pump OUT went directly to fuel filter and out to engine
The 3rd port on filter went back to gas tank and the FPR
Fuel pressure at the engine end was way more stable with this setup
Extreme long shot, lol

I've always wondered why they went to the 3 port fuel filter in the early 2000s, this makes complete sense, I don't want to convert the truck to the newer system but I may consider it. (but still don't want to do it)

 

[holyford86]

A bit of an update, noticed that the egr valve was occasionally receiving vacuum at idle, changed the EVR and the occasional stalling along with what could be described as a chug at part throttle while cruising went away but idle would still drop to around 450 rpm occasionally. I had a full kn intake on the truck, the air filter would fall off occasionally because the adapter was a piece of trash, ended up swapping back to the factory intake which I had saved. Now the idle now stays slightly above 500 rpm at its lowest point, I've put a few hundred miles on since and it seems fine now. Apparently the maf was slightly upset with the air flow or something, it's all I can figure.