95 850 Turbo with Extreme Engine Surging

[Nick777]

Ok, so the symptom is that my car can be idling normally, or my engine can be responding predictably (meaning more throttle results in higher rpms). However,....

It often occurs, both while idle as well as while driving, that my engine rpms surge... and it some cases i mean SURGE (from idle to 5000 rpms has occurred, though not usually this bad). As you might imagine this is potentially incredibly dangerous if it occurs while driving. Anyway,...

I KNOW that the Coolant Temp sensor (or its connection) is bad. Now i know that's confusing the fuel injection system, but i dont imagine that alone is causing this crazy surging issue.

Here are my thoughts:

1) Throttle leakage/sticking - don't think so based on symptoms appearing without playing with throttle.
2) IAC issues - don't think so because, really, can an IAC even open up enough to allow sufficient airflow to cause surging from idle to 5k+ rpms?
3) Vacuum leak(s) somewhere? I dont think so because isnt the only fuel injection "modulator" downstream of the MAF the pre-cat O2 sensor? And does that O2 sensor even have the capacity to modulate fuel delivery enough to send a normal idle to 5k+ rpms?

Sooooo.... my thoughts run along the lines of this --->

1) there MUST be leaking air coming in from somewhere, possibly from multiple locations, or else the engine couldn't possibly ever surge as high as it does and,...
2) there MUST be some involvement with a major sensor involved in modulating fuel delivery (not the O2 sensor, but i suppose that could be bad as well), like the MAF and TPS. Of course, there is the ECT sensor issue, which we know about already.
3) possible partial involvement of *Multiple* issues of the types stated above. Even if one of those above stated possibilities alone can't cause the surging, probably if multiple issues are contributing, they could.

If you've made it this far, thank you for your patience!!

I'd love to read your thoughts!

[erikv11]

I would

(1) Fix the ECT. Go junkyard if you don't want to pony up for the OEM one.
(2) Check everywhere for vacuum leaks.
(3) Unplug the MAF, start it up, see how it goes with the MAF unplugged.
(4) Clean the throttle body if/when I could talk myself into it. Clean the IAC at the same time.

Does this car have OBD2? Can you hook up a reader and get data while it is surging?