Hi Bill,

I'm sorry you are having problems with your 1991 Volvo 740.

I would keep an eye on air intake (vacuum) leaks.

To the unammed poster:
- Indeed, not all 700-series Volvos came turbocharged in North America.
- A 1991 Volvo 740 normally aspired with Bosch engine management uses the Bosch LH-Jetronic 2.4 and Bosch EZK-116 (?) ignition with the RPM or Crank Position Sensor. (Many sensors have more than one industry term. AMM or MAF?)

Bill, have you checked both the Bosch LH-Jetronic (fuel injection) on OBD code reader socket six (ignition, EGR if equipped [Germany, CA-US])?

You are probably well informed on the 700-900-90 series GAQ hosted right here on your brickboard.com.

How to run OBD:
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

With instructions on how to force the OBD to test connected sensors over and above the press the button once and hope for 1-1-1.

The OBD-1 is fickle and not always reliable. Like as is written in the OBD web page, you can have sensor fail and a fault code may never be set.

Idle problem symptoms and causes, throttle body:
https://www.brickboard.com/FAQ/700-900/EnginePerformanceSymptoms.htm

However, problems such as these start intermittently. Like the TPS or CPS and ...

Your Throttle Position Switch (TPS) can begin to fail. That switch, on the LH-2.4, is a switch, only. If adjusted and working properly, with foot off the pedal, the idle air control system engages. As the TPS fails, it can actually engage the idle control system while you have your foot on the pedal. The symptoms, though, are more of a bogging down of the engine as you accelerate. Though TPS failure symptoms can vary.

Bill, what you describe may be something that has action on the idle. I'm thinking you run regular (not synthetic) oil in the B230 engine of your 1991 Volvo 740.

With the engine OFF, can you check the action of both the throttle by turning it in such a way that would increase RPMs. Run the thottle action back and forth a few times.

I ask what kind of oil you use as carbon-build up of carbon combustion products forms inside the air intake system. The B230 throttle body like to be clean on occasion, and the gasket replaced.

I may suggest you look at the fuel injector rail fuel pressure regulator (FPR). The FPR is at the front of the engine on the fuel rail. A vacuum line connects between the FPR and a spot on or near the throttle body.

Pull the vacuum line? Any fuel comes out? Replace the FPR. Though a leaking FPR may not cause a racing engine as you describe.

I believe it is your throttle body. You may have a throttle linkage problem, perhaps? Though unlikely.

I feel it is your throttle body assembly. It may be dirty, the vacuum line ports are clogged, and you have much carbon build up, preventing free butterfly throttle valve movement.

https://www.brickboard.com/FAQ/700-900/EngineFuelinjection.htm

You can try using some sort of Gumout carb cleaner. Yet the better bet is removing the TB, cleaning it as directed in the FAQ and your Volvo OEM Green Manual (find them online for FREE), your Haynes manual, and I dunno. Clymer or Chilton.

Else, I'm certain others will chime in with all sort of usefulness for you.

Does that help?

Questions?

Hope that helps.

Thank you,

Duffy's Tavern.