Lots of idle issues on this forum but can't find one similar to my experience.

88 ranger. 2.9L manual.

Just bought this truck. It ran great when I bought it. After a day or 2 it would idle rough. Almost die but it wouldn't. No check engine light. It seemed to do this intermittently. Some times I'd start it and it be fine. Other times I'd start it and I'd expect it to die at each stop light.

Now for the last couple days it dies on every start up. It won't idle. It'll start right away and then die. I can keep it going by keeping the engine rev'd but the check engine light is one. If I keep it running and start driving down the road it'll drive with the check engine light on for about 5 minutes. Then the light goes out and it runs like normal, even at idle, until I shut it off. Then it's the same story at next start whether it's right after I shut it off or hours later. It doesn't seem to matter whether the engine is hot or cold. When the check engine light is out, it runs great the rest of the time whether at speed or at idle.

To me it sounds like a sensor is on it way out. I figure if it was vacuum, it happen at every idle, and if it was fuel it happen all the time or at least every idle. Any ideas?

Tried to get a code at parts store but the reader wouldn't work. It couldn't tell it was plugged in the the truck for some reason

 

Why not start with a good visual inspection ?
Check for any vacuum hoses that may have come off since it sounds like the computer is able to compensate after it has run a while. Since it is new to you the condition of the usual things like spark plugs, coil, wires etc. are unknown and can affect performance.
What if anything have you checked so far?

Most stores don't have the equip to check OBDI systems and that is what is in this vehicle.

A coolant temp sensor may be telling the computer the engine is warm all the time resulting in a lean mixture that is fine for the engine when it is warmed up,but not when cold.

 

That's sort of my plan. I got some stuff for a basic tune up. I've been side tracked though wiorking on anither vehicle while I had a friend with certain equip available to help. I post what I find when I get back on the ranger.

I hadn't thougt about the coolant temp and the iac looks pretty new on here from the last owner.

 

Possible fuel related issue? Have you replaced the fuel filter(s) and checked the fuel pumps for operation? The 1988 was the last year for Ford to use a LOW PRESSURE pump in the tank that then feeds a "HIGH PRESSURE" pump on the frame rail. Also, the 1986-1987 Rangers/BII's used a canister filter in-line with the high pressure filter. The canister filter was phased out for the 1988 model, but the canister was still in place as it was needed for the fuel return line. However, I bought new an early build 88 Ranger S/C 4x4 w/2.9 that had a filter in the canister, so you may have one there or somebody may have put one in. I removed mine as it wasn't needed. The 1986-1988 Ranger/BII's used a dual circulating fuel system. The low pressure pump fed the high pressure pump, then the fuel return from the injector rail fed back into the canister assembly. Have you checked for fuel in the vacuum line on the back side of the fuel pressure regulator? If there's fuel in the line, then the regulators diaphram is breached (common) and needs replaced.

 

Update on truck
now that i finally got a chance to dig in the other day

I was sure a sensor was bad because if kept tryin to start it and wiggled the wires at each one betweens starts, eventually i'd get a "good" start with out a cel and bad idle so i assumed i had a broken wire in there.

I gave it a basic tune up as an excuse to go digging around. Plugs, wires, cap rotor, fuel and air filters plus some valve cover gaskets because it needed them. I took the intake plenum off to make room and replaced those gaskets. Cleaned the iac and throttle. Both looked good.

However i found a couple spark plugs that were loose(or just barely hand tight) And all of the plugs had a wide gap. The manual says .44 but they were all at .55-.65. They looked good other then the gap. After i put it together, no more light or bad idles.

My theory is the hand tight plugs were seated just enough that when the engine warmed it would expand just enough on the plug threads to close the leak. That mixed in with the wrong gap kept it running bad. I don't know if thats's what it was but that the only thing i could find. Maybe the valve covers were leaking oil onto the plugs to affect them.

Since it idles now i could confirm no vacuum leaks and i didn't find any broken wires.

I was supprised it was fixed from just a tune up.

 

While I said no more bad idle, it still has a very slight hesitation at idle once in a while. Usuall when warm. Not at start up. It's not much. It idles at 900. It may dip to 800-850 at a stop light but not everytime? Does that indicate anything to anyone?