Hello,
I have an '89 pickup with the original 3.0L motor. I've been having head gasket issues, but I don't think they're related to the surging at this point.
It idles fine without input, and 2000 RPM and up it has no issues, but when I try to hold the idle at 1500RPM it surges back and forth twixt 1000RPM and 1500RPM.
I've replaced the entire coolant system with an HPS kit and a pump from a junked '92 pickup, I installed a short ram intake from HPS and put in a new(er) MAF from a totaled '91 4runner with it.
5 of 6 spark plugs are new, as well as the spark plug wires and distributer. I'm gonna work on the 6th plug with a friend, a torch and some beer later today.
I've ordered a new ignition coil but I'm not sure that's the issue. The only thing I can think of at this point is that the MAF doesn't know how to compensate for the short ram now that the resonator and silencer are gone, but I don't understand why it only presents at 1500 RPM.
Any thoughts or advice before I drop $200 on a part that might not be necessary will be greatly appreciated. Thank you!

 

I'm no expert, but I would start with testing the AFM, using a good quality analog ohmmeter. If that test good, I would check voltage at the ECU per the ECU test in the repair manual.
I just found a problem with my AFM, by testing voltage at the ECU.
I've been chasing a couple of problems on my truck, and I have learned some about the system.

 

Thanks for the response!
Turns out that mechanic was just incompetent. My friend and I removed that last spark plug in just under two minutes. We think he just wasn't using a universal joint to get at the plug. After we got that done I replaced the starter coil and it seems to run fine now. The plug was super cruddy (Probably from the headgasket issue) and must've had a weak spark. Or maybe it was the coil. Either way, one problem out of a million, down.
I'm still going to go through with your suggestions, though. I like being proactive.

 

Glad to hear you got it sorted out. Something else I figured out on my truck is checking the O2 sensor. The factory manual says to check it at the test point across terminals EF1 & E1, and look for 8 or more fluctuations in 10 seconds, if I remember correctly. It doesn't give voltage amplitude. From online research I found that is a "conditioned" signal (amplified, apparently). Mine fluctuated, but very little. Measure the direct O2 signal, OX terminal at test point or at ECU, and E1, I believe it was. I also found out that my analog multi-meter would not swing the needle fast enough to see amplitude. With my digital meter I could tell it was swinging from .25V - .73V, at least. I think a multi-meter with a show min max feature, or an oscilloscope is what is really needed. I'm calling mine good enough for now, it's been running in open loop for a while and the O2 sensor has probably gotten a lot of soot on it, maybe it will burn off now that it's not running so rich.

 

Is everything good now?

Edit: Seems yes