Rough Idle After Starting And Sometimes Dies

[RonD]

900rpms is a bit high for warm idle

Manual trans should be about 650, automatic 750(in Park, 800 in gear)

After engine is fully warmed up, let it idle and unplug the IAC Valve
RPMs should drop to about 500-600, engine barely running
If it does not drop then you have an air leak, vacuum leak
With IAC Valve still unplugged, pull off one vacuum hose at a time from intake and plug its port with finger
When RPMs drop you found the system with the leak, follow that vacuum hose
Start with power brake booster hose and PCV hose


If RPMs do drop to 500-600 then no vacuum leaks, and its the computer(PCM) that is causing the higher idle
Hard to fix that, if you want you can pull out the PCM, open it up and have a look at the 3 blue capacitors, they leak after 20+ years and can cause odd issues, under $5 to replace them just to be sure, and free to have a look
It's also good to have a look at the PCM connector, for corrosion on pins, and unplugging and plugging back in ANY connection cleans it off and can make for a better connection, that applies to all connections in the vehicle
All the sensors run on 5volts, and 0-5v is the Return signal to the PCM, so a dirty connection will cause a lower voltage and incorrect sensor data

 

[RolandTrafton]

I’m hard pressed to find a vacuum link, but ordered some hose (of various inside and outside diameters, if anyone knows the specific size that could be helpful) anyways to swap out as many as I can, hopefully all of them.

But if it is the Computer, would it be as simple as ordering this replacement Cardone Brand ECU from Rock Auto and swapping it out? Seems like a big deal to swap out the automobile’s brain?

1987 FORD RANGER 2.9L 177cid V6 Engine Control Module (ECM Computer) | RockAuto

RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.

www.rockauto.com

Update on the Seafoam Trial: Not really noticing a big difference. Liquimolly Injector Cleaner and Liquimolly Valve Cleaner happening this weekend.

Temp Sensor and EGR Valve swaps also happening this weekend. New Fuel Injectors and Fuel Injector Pressure Regulator happening next week.

 

[JerryC]

I've been there... I wish I could point to one thing that made it better but it was sum of many repairs and parts that all together got it running smoothly.

Looking at the thread I don't see where you cleaned or replaced the ACT, it's on the passenger side of the intake manifold. Mine was covered in what looked like tar. Check the O2 sensor connector and the O2 sensor itself may be bad. While on the topic of O2 sensors, an exhaust system leak before the O2 sensor can affect the O2 sensor readings.

Vacuum leaks, I've had a few, the upper intake gasket was a big one and the grommet at the brake booster was a small one. With couple of dru rotted smaller hoses in between. Clean the throttle body if you haven't, they make a special throttle body cleaner, use it. Gunk could be making the plate sticking open a little.

Grounds, make sure you have them and that they are good. The one that gets lost is usually the one from driver side cylinder head to the firewall. Why is this important? Because electronics need a solid reference to measure voltage and with bad or missing grounds that reference is no longer uniform through out the vehicle.

If don't have them already get the shop manual and the electrical and vacuum manuals. They are usually available on ebay for cheap.

 

[aball4620]

Sounds like your 87 is about as cursed as my 88. It does sound like a vacuum leak but possibly a rotted out wire that is getting better continuity with a hot engine bay. OBD1 engines have something called a “wiggle test” - seriously it’s in the reader manual. Basically you warm up the engine, hook up the reader, and start wiggling parts of the harness. I found a couple questionable wires on mine when I did it on mine.

Have you checked codes yet? Or was that the reason for throwing all the parts at it?

For the computer they rarely go bad but there are a couple things that can happen. Pop yours open like RonD suggested. Mine only has one ass opposed to the three capacitors he mentions, but it is a quick thing to check off your list. This body series can also let water leak down onto the ecu so check for dampness. When I replaced mine with a rebuilt from cardone it changed exactly nothing.

I’ve replaced everything you have plus a few more, so I’m interested to see if you figure this out.

 

[aball4620]

Manual trans should be about 650, automatic 750(in Park, 800 in gear)

So Ron while still changing the stumbling issue on mine I’ve noticed that my auto in park is a hair over 800 and in gear drops to about 700. Thoughts on that?

Not trying to hijack the thread in anyway, but vacuum leaks are sometimes hard time find and might be helpful

 

[RonD]

That reads like the Computer is not "seeing" the Gear selection from the NSS(neutral safety switch) on the transmission

NSS on the A4LD is above the shifter on drivers side of transmission

Uses computer pin 30

The computer should bump up the idle when in R or D 2 1, or when AC is on

Not a big deal as long as engine doesn't stall when you put it in gear, not a vacuum leak issue
Some one may have adjusted the anti-diesel screw on the throttle linkage to set the 800rpm idle because engine was stalling when put into gear when the pin 30 setup failed to work