Another Renix with an idling problem

[Bobolink]

I have a situation that it seems just recently started. The 90 Renix 4.0 will not idle. I sold this truck to my brother in mid December, and the problem started after he bought it, and because I’m feeling bad about it, I have it back to try and figure out what’s wrong. I knew of no problems at all before I sold it, and I never had any real problem with it for the 25 years that I had it since new.

 

I have done everything that I can think of. The grounds and connectors are all clean and sound, all cruiser’s tips are accomplished long ago and just rechecked. I have a new NTK O2 sensor, and the voltage checks on that, oscillating between 1-4V, new MAP sensor, and its voltages are all to spec. The TPS has 4.99V to it and is adjusted for the engine (rather than the tranny) at .849V. Distributor is reman, maybe 4-5 years ago, and it's been indexed. Vacuum readings are normal (until it decides to die). IAC is new. Temp sensor readings are to spec (but only checked when cold). It has a new Bosch fuel pump, and fuel pressure shows 31PSI with vacuum, 39 without. A new ICM. I replaced all the relays. And the EGR is removed, and blocked off, so that’s not a factor. Plugs look good, and they, along with plug wires were new a couple years ago. Distributor cap and rotor probably a couple years old, too. The injectors are the four hole 746s and have been there for a few years. Throttle body is clean as a whistle, and I’ve done the Seafoam treatment, dribbling it through the throttle body at fast idle. Vacuum lines are checked and good.

 

It seems that the truck idles fine when cold at around 900 RPM, in open loop, but starts to have trouble as it warms to operating temp, and the idle RPMs start to settle down to 750. Then once at operating temp it usually does fine, too, with hot idle at around 600-650. Once the temp gauge starts to move up, I can expect the idle to start getting a little rough, and might stumble and recover a time or two, or simply quit without a stumble. And although it usually does fine when hot, it has died once or twice at operating temp. Regardless of temp, it always restarts without a problem. If I am driving to town from my place (a 20 mile trip) I have no problems, because I have no occasion to idle till I get to town. That may be why my brother's the one who started having problems. He lives two miles from his work, and has several occasions to have to stop. It seems to me to be more apt to die in neutral, than under the slight load of having the A/T in "Drive".

 

Truck runs good, gets about 18 MPG or so, exhaust smells slightly rich (when compared with modern engines), but not overly so, and plugs readings don’t show any signs of too rich a mixture. Compression is good for 265K miles, 135 to 142 on all 6 cylinders.

 

I haven’t yet checked the resistance values on the temp sensors when hot (only checked ‘em cold), so guess I should do that. What else is there?

[cruiser54]

You have the sensor values? 

[Bobolink]

MAP sensor, plugged in and back probed, 4.4V engine off, key on ... 1.72V hot idle. Both temp sensors, as I said only before start with engine cold, ambient temp @ 78° 3000Ω. Still gotta check the ohms at various hotter temps. Don't know what other values to give ya, other than what I've said. I'm taking the correct values for reference right out of the 1990 FSM that I bought new along with the truck (the book's as well used as the truck, believe me :thumbsup:) 

 

By the way, before you ask, I did pull the throttle body to closely check the throttle plate, in case brother TIm messed with it (he's kinda like Uncle Bob, don'tcha know). It's closed. 

 

I got those readings from both ICMs also, that I'll post in the other thread. Not much to tell there, either.

[cruiser54]

Check the sensors at a known coolant temp. 

 

Couldn't hurt to also remove the IAT and carefully clean the tip with carb cleaner. 

[Bobolink]

Well, the ol' truck is runnin' good as new again. Idles good, runs good. Seems it was the coil ... a new one fixed it. Must've been breaking down. One bonus out of this ordeal. I replaced the O2 sensor several weeks ago, and the mileage has improved condierably as a result. Gone form 18.5 MPG avg, to over 20 MPG. I also found that it seems to idle and run a bit smoother with the TPS set for the transmission, rather than the engine. When I set the correct voltage (4.15) on the square four pin connector, the voltage reads considerably lower on the flat three pin connector than it would as set for the engine. It reads .65 volts, instead of the .83 volts that would be the setting for the engine. But as I said, it seems to like that setting better.