1996 Ford Ranger 2.3L I4 XLT 295k mi
Hi guys, my idle is pretty smooth since the new timing belt, and crank repair (see my last post). However, the idle does not always return when I decelerate to a stop while coasting in neutral. *I had this issue before doing the timing belt for about two years now* Sometimes the rpms will drop to idle but very slowly, other times the rpms will stay higher than idle even after stopping.

I have observed that I can 'force it's to idle correctly by loading the engine (1st gear, foot on brake, slowly letting out clutch to bite point). After loading the engine, I can shift back to neutral, and it is back to normal.

I have replaced IAC, TPS, MAF, IAT, And virtually all Vaccum lines. What would cause this?

 

Emissions requirements has idle sitting above 1,000rpms until Speed is under 5MPH, so you may be fighting against the Law, lol

In a 4cyl, it may be up to 1,200rpm minimum until speed is under 5mph
Engine idle(under 1,000rpm) is a big polluter since it has to be a richer mix to prevent overheating, this is why the requirement is in place

If manual trans then when shifting, RPMs should also hold higher for at least 7 seconds with clutch pedal down, before they start to drop, another Emissions thing

And just a heads up with a manual trans
One of the biggest MPG boosts with Fuel Injection is the computer's ability to SHUT OFF injectors when coasting
So when coasting in gear with foot off the gas pedal and RPMs above 1,500 or so, computer shuts off injectors, until clutch pedal is used or RPMs get lower
In the old days with Carbs, coasting in Neutral was better MPG because the higher vacuum in intake when coasting in gear sucked in more fuel from Idler Jets
No so with fuel injection, coasting in gear will have better MPG



If idle is high when stopped then do this simple test
Have engine warmed up and idling
Unplug the 2 wire connector on IAC Valve
Idle should drop to 600rpm or so, or engine may even stall, this means no vacuum leaks, but also means Computer was setting the idle RPMs as programmed
If idle doesn't drop then computer can't set a lower idle because of a vacuum leak usually, but can also be someone has adjusted the Anti-diesel screw(looks like idle screw)

 

A previous mechanic messed with that screw. How does one tune it?

 

Warmed up engine, idling
Unplug IAC Valve
If engine stalls turn screw clockwise 1 full turn, re-connect IAC Valve, restart, repeat test, you want to end up with barely running WARM engine with IAC Valve unplugged, for a 4cyl that will about 600-650rpm, 4cyl engines vibrate A LOT below 750rpms, so 750 is about the lowest it should be with manual trans, 800 on automatic
V6s and V8s can have smooth idle down at 625
4cyl only fires every 180deg of crank rotation so hard to get a vibration free idle, lol

If RPMs do not drop with IAC Valve unplugged, turn the screw 1/2 turn counter-clockwise, RPMs should drop a bit, if not turn it back to where it was, Anti-diesel screw is NOT the issue
If RPMs do drop, then do another 1/2 turn, until warm engine is barely running, its set, plug back in IAC Valve and RPMs should go up to Computer's programmed RPM setting

 

I adjusted the screw back to where it barely touched the stop plate. This practically fixed the problem. However, the idle is just a smidge on the rough side compared to before. Also, there was no effect when the IAC was unplugged. The rpms jumped and resettled when I plugged it back in. I'm thinking Vaccum Leak. But I tested the intake for leaks using the cigar, and the disposable glove method. There didn't seem to be any leaks.

I know that the exhaust has a leak near the manifold on the egr tube. But I don't think that would cause this problem as the EGR is normally closed during idle.

What else would cause this?

 

Whats the age of PCV Valve?
Should be changed every 3 oil changes, or at least cleaned, i.e. soaked in gasoline/solvent to get rid of the oil inside so valve can move freely

Put a can of Seafoam, or similar injector cleaner, in the gas tank, see if that cleans up rough idle, I need to do this once a year on my 1994 4.0l, all original sensors, controls and injectors
(well new O2s every 10 years or so, lol)

 

O2 Sensors are about six years old. The fuel injectors are within two years old, and the PCV valve was replaced back in 2019 (3 years). The gasket which contains the PCV valve is original and is kinda loose but it seems to seal it so I left it alone. I put upper end lubricant in the gas tank (LucasOil) and engine stabilizer (LucasOil) in the crankcase.

 

Clean PCV Valve
Try Seafoam in the gas tank

 

I'll give it a shot on Monday. More double shifts! 😆

Truck is driving great on the freeway, with more power than it's ever had since I've owned it (9yrs) However, now that I adjusted the anti diesel screw, there is this weird place between no throttle and lightly tapping the throttle where the truck will accerate for a second, immediately fall back, then jump forward again as the throttle is slightly increased. It doesn't do this when I press down firmly into the throttle while merging onto the freeway, just while cruising at low idle. The bucking really throws me back in the seat! Thankfully truck is manual tranny, otherwise I'd be rear ending some peoples, I just pop the clutch back in! 😁🤦 All other acceleration is fine afterwards. Will the anti diesel screw cause this behavior, or should I be looking at a different sensor?

 

The TPS(throttle position sensor) voltage was changed when you adjusted the anti-diesel screw because that screw does move the throttle plate shaft which is what the TPS is connected to

You have a 1996 Ranger so can use an OBD2 reader, a Bluetooth OBD2 reader is under $20, if you don't have a reader now, these work on ANY 1996 or later vehicle, not a Ranger or even a Ford thing, ANY VEHICLE, so a good tool to have

Key on engine off
TPS should show 0.69 to 0.99volts on center wire, under 1 volt, on an OBD2 reader that would be 17 to 19%
TPS, like most sensors, uses a 5volt scale
1volt would = 20% of 5volts, so just under 1volt, 19%
At WOT(wide open throttle) center wire should show 4.5-4.6volts, on OBD2 reader that's 90-91%, 4.5v = 90% of 5volts

But before testing I would reset Computer, with OBD2 reader or unhook the battery for 5 or more minutes, this clears computers "learned" operating parameters
The computers "learns" things like IAC Valve voltage for a specific RPM and TPS range, i.e. voltage with throttle closed, so it has a starting point
So you could have an older starting point in memory, like 0.99volt, and with adjustment to anti-diesel screw its now 0.79v, so confusing to a "black and white" device, computers are very literal, lol

Some TPS units have slotted bolt holes, so they can be adjusted to set output voltage on center wire, throttle closed, and WOT both need to be "in range", under 1v and at least 4.5v at WOT

 

Thanks for your knowledge! I honestly wasn't expecting this caliber of assistance.

As for the reader, I have a $160 innvoa scanner with live data. It does most things, although I wish it were bi-directional. The PID for tps said about 15.6% and would kinda flutter between 15.5% and 15.6% during idle before I adjusted it (old wires probably?). I don't know what it is now. The TPS is brand new (only a month old). It does not have slotted mounts. I should also mention that I disconnected the battery for 2 minutes after adjusting that anti diesel screw.

Yay! More voltage tests for my voltmeter! Getting excited for Monday!

 

TPS should be at least 17% throttle closed, test it again , also test what WOT shows, should be 90%, test with engine off

Loosen TPS screws and see if you can turn it enough to get 17%
Worst case, turn anti-diesel screw in just enough to get 17%

Reset computer with OBD2 reader, not just clear codes, "reset"

 

Yesterday, I gave it the good ol' seafoam treatment to the intake, cleaned the PCV valve, reset the computer, and adjust both the TPS position, and anti-diesel screw to read 17.3% (this was as close as I could get it). Idle is still a bit rocky. However the idle speed is now +/- 15rpms, which is nice because it doesn't jump around as much as before. Shifting is also noticably easier; I have worn out synchronizer on 2nd gear and it used to grind as I shifted on acceration unless I rev matched. Now it only grinds if I'm slamming it in. Also a nice improvement!

I hear a hissing noise (sounds like a vaccum leak) coming from EGR Solenoid (Replaced 3 months ago), but all the lines are properly connected. Is it commonplace for these solenoids to make this noise when at idle? Is it like a pressure bleed off?

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No, not normal

Unhook the vacuum hose from EGR Solenoid to EGR valve, at the EGR end and plug it with a screw/bolt
Start engine see if you still hear the leak

If so then unhook the vacuum IN hose at the solenoid and plug that hose with a screw/bolt
Start engine and see if you still hear the leak
If not does engine run smoother that way

Replace EGR solenoid if its leaking
You can try reversing the hoses on the solenoid, it shouldn't matter but CAN matter on some solenoid valves