97 4.0 idle surge and tach jumping. no codes

[sostahoe]

This has been ongoing for awhile, here is a quick history. original probplem was no start and replaced coil pack, good fix. 2 weeks later no start, -replaced coil pack and no fix, replaced crank sensor, good fix. Owner is now driving daily, but has intermittent idle surge that has been ongoing for months.
Now I am asking for help troubleshooting the surging idle.
It will surge from good idle too as high as 3500 and down too near engine die. (has even died). It will idle and run without surging while not in Park or Neutral (Loaded engine or driving). Also noted that randomly the tachometer will bounce without relation too the engine RPM, sometimes as much as 4-5 thousand above current engine RPM and then return to normal again. I have cleaned IAC valve. Removed and re-installed crank position sensor. I am getting no codes. I sprayed for vacum leak check and no rpm changes. I may be seeing incorrect coolant temp gauge readings as they seem way lower than I expected, but owner of vehicle says it has always been that way.
Any suggestions ????

 

[2000StreetRod]

TPS voltage?

It might be a noisy TPS. The PCM reads the TPS output to determine throttle position. With ignition On and engine Off the TPS voltage should be approximately 1.0 volts. As the throttle is opened, the TPS output voltage should increase smoothly to around 4.5 or more volts at WOT. If the TPS output is good you may want to perform an idle adjustment.

 

[chapsdeva]

I had stumbling and surging when my IAC went out, but the surging wasn't near as bad as you are experiencing (mine was 1500-1700 rpms). Cleaning it didn't help and it took a new one to fix the problem. Yours sounds more like a TPS issue, but wanted to give you the fyi that cleaning the IAC doesn't always work (Although should be tried first).

 

[Popjack]

Sostahoe,did you ever resolve your issue here?What you have described is exactly the
problem I am having on my 92 Aerostar.Been considering engine temp sensor as possible problem.