1994 Vovlo 940 NA (208K miles):
Battery, fuel pump, and fuel pump relay replaced several months ago.
Already cleaned the MAF and throttle body.

Replaced the crankshaft position sensor (CPS) with the cheapest aftermarket product I could find. That solved the no start and no tach movement issue. Now that the engine starts, I can see that I have an high idle issue. Plugged up all holes in intake hose, etc. that I can find, and am considering the Idle Air Control valve may need cleaning or replaced. However, I may still have an issue with the new CPS. Here's what's happening: Starts fine first time in morning; runs with high idle (1200 up to 2000 rpm); can put it in gear and drive for a couple of minutes, then it dies. Or, can let it run for a minute or so, put it in reverse, and it dies. Then, cannot start it right away and there is no tach movement (indicating ECU is not receiving CPS signal). After it sits for a while, presumably once it cools for a bit, it will then start again.

Is the cheap CPS the intermittent start problem? Is the new CPS good, but something else may be causing the CPS to overheat and temporarily not send a signal?

Can the IAC be cleaned vs replacing it?

There seems to be more than one issue going on, so trying to diagnose and get it running without spending a lot more money.

Owner can no longer do their own mechanic work, so ready to sell. This is a redblock engine, so many more miles left on it if we can get it to start and run stably.

All experienced suggestions appreciated.

 

It sounds like you're dealing with a tricky situation with your 1994 Volvo 940. Given that you've already replaced the CPS and cleaned the MAF and throttle body, it’s possible that the cheap aftermarket CPS is causing intermittent issues. These sensors can sometimes be https://tropical-casino.com/ unreliable, so it might be worth considering a higher-quality replacement if you suspect it's malfunctioning.

 

The hot stalling problem and the lack of a tachometer signal are most likely due to the low-end crankshaft position sensor, which may work when cold but fail when hot; the high idle is a separate issue, usually related to a dirty IAC valve or an air leak, so start by cleaning the IAC and checking the hoses, and replace the CPS with a quality model to stabilize starting and running.