High Engine RPM while driving

[volvojj]

Hello I would like to ask for any ideas on how I may fix the following problem on my Volvo 940:
- 2-3 minutes after starting the car, the engine RPM increases although I do not press on the gas pedal. It can happen while the car is idle or while driving. When driving it usually happens once (95% of the time), although it might happen twice (and in very rare instances 3 or more times). Usually the RPMs increase up to 1750-2250, and normalize after I brake and shift the gear (2-3 times)

- So far the following repairs have been made to the engine, although the problem still remains:
1. cleaning the throttle body
2. installing new throttle position sensor (TPS sensor)
3. installing new idle air control valve (IAC valve)

I would appreciate if you have any ideas about what might be the problem or what is the best way to trouble shoot and detect what is the issue?
Kind regards
ps. My mechanic says that he does not think it is a vacuum leak (although I am not excluding that possibility).

[360beast]

After fitting the new TPS was the base idle setting setup correctly?

[RollingThunder]

I agree with 360beast, I'd be looking at the base setting for the throttle position switch and checking the accelerator cable adjustment.

I'd also check the wiring to the coolant temperature sensor, and I'd try unplugging the MAF to see if the problem continues with the MAF unplugged.

You don't mention a ragged/hunting idle, so I'd agree with your mechanic in that it doesn't sound like a vacuum issue to me.

[StrongSpearWorks]

I'll second checking the ECT sensor and wiring. A dying / misreading engine coolant temp sensor can issue all kinds of odd behaviours.

[TonyS9]

This is a weird software thing, it happens when you use full throttle or possibly large airflow when the VSS (vehicle speed sensor) signal isn't available. Have you checked for engine fault codes, it would normally record this but will not light the Lambda light.

Is your speedo working? If so then you need to check the speed signal is getting to the fuel ECU. It doesn't need to be accurate just some pulses.

We get the same problem when engine swapping to the 360, but it is fixed by installing the 440/Golf speedsensor to the 360 speedo.

Year, model, Engine type is always useful.

[volvojj]

Thanks everybody for your answers. Here are my replays to your posts:

To: 360 Beast - Yes, I have shown the mechanic some of the YouTube videos regarding this procedure (setting a base idle on Volvo 940) and he was familiar with what I was trying to show him. Also, when in idle the engine runs 750 RPMs.

To: RollingThunder
- Regarding the wiring to the coolant temperature sensor - I will explain to the mechanic to check for this. I have additional question here – from experience do you think that it is more a problem with the wiring (and what kind of a wiring problem can it be?) OR with the coolant temperature sensor (faulty sensor?). If it is a faulty sensor will the car computer register a faulty code for this?
- Regarding removing the MAF to see if the problem continues – If there is no problem after removing the MAF, what does it mean? Does it mean faulty MAF?

To: StrongSpearWorks - I will explain to the mechanic to check for the ECT and wiring.

To: TonyS9 –Regarding your question about the speedometer - it is working. May I ask what do you mean by “…you need to check the speed signal is getting to the fuel ECU…”? How is this done? Is it done with computer software?
I rarely use full throttle, at most 3000 RPMs, most of the time around 2000 on open road. Have not checked for faulty engine codes (the mechanic’s software could not connect). I am trying to organize somebody else to have a look.

Kind regards
The model of the car is 1996; 2.3; 99KW; B230GK.

[StrongSpearWorks]

Originally Posted by volvojj

Thanks everybody for your answers. Here are my replays to your posts:

To: 360 Beast - Yes, I have shown the mechanic some of the YouTube videos regarding this procedure (setting a base idle on Volvo 940) and he was familiar with what I was trying to show him. Also, when in idle the engine runs 750 RPMs.

To: RollingThunder
- Regarding the wiring to the coolant temperature sensor - I will explain to the mechanic to check for this. I have additional question here – from experience do you think that it is more a problem with the wiring (and what kind of a wiring problem can it be?) OR with the coolant temperature sensor (faulty sensor?). If it is a faulty sensor will the car computer register a faulty code for this?
- Regarding removing the MAF to see if the problem continues – If there is no problem after removing the MAF, what does it mean? Does it mean faulty MAF?

To: StrongSpearWorks - I will explain to the mechanic to check for the ECT and wiring.

To: TonyS9 –Regarding your question about the speedometer - it is working. May I ask what do you mean by “…you need to check the speed signal is getting to the fuel ECU…”? How is this done? Is it done with computer software?
I rarely use full throttle, at most 3000 RPMs, most of the time around 2000 on open road. Have not checked for faulty engine codes (the mechanic’s software could not connect). I am trying to organize somebody else to have a look.

Kind regards
The model of the car is 1996; 2.3; 99KW; B230GK.

The 25+ year old EC1 connector that lives next to the block and under the manifold can get brittle with time, as can the wiring. Mine was so sloppy it wouldn't make a useful connection any more. I snipped it off, spliced on a new one and put heat shrink over it all. Fixed that issue that time.
The ECT sensor is a known weak point in the LH system (any VAG owner of a similar vintage will tell you the same &#129315. Sometimes they last for 20 years, sometimes 18 months. When they fail they can produce all kinds of odd behaviour, even if they are apparently working (resistance and voltage are in range at a sample of temperatures). If it does transpire to be the sensor that's faulty, get a Bosch one; they're more expensive, but experience has taught me buy cheap, buy twice.

[RollingThunder]

Originally Posted by volvojj

To: RollingThunder
- Regarding the wiring to the coolant temperature sensor - I will explain to the mechanic to check for this. I have additional question here – from experience do you think that it is more a problem with the wiring (and what kind of a wiring problem can it be?) OR with the coolant temperature sensor (faulty sensor?). If it is a faulty sensor will the car computer register a faulty code for this?
- Regarding removing the MAF to see if the problem continues – If there is no problem after removing the MAF, what does it mean? Does it mean faulty MAF?

There are two coolant sensors, one for the temp gauge in the dash and one that the ECU uses. Both are located under the intake manifold near the IAC, hence me wondering if the wiring had been disturbed when the IAC etc had been replaced? The wiring is old and brittle, so any excessive movement of the cables could have cracked the cable strands inside the insulation. As the sensors themselves are hidden under the manifold, I wouldn't expect there to be any physical damage to the connectors. Nevertheless, it is possible that a sensor has failed, or at least degraded. I've just ordered both for my 940 as a preventative measure. They are available still, but are now heritage parts.

If the ECU is receiving incorrect temperature signals, then it will calculate the fuel required by the engine incorrectly. When the engine is cold, the ECU sends more fuel by opening the injectors for longer (equivalent to using a choke on an old carburettor car), and when it is normal/at running temperature, it uses a 'normal' amount of fuel - equivalent to 'choke off'. So if your temp sensor is reporting a cold engine to the ECU, it'll be using a level of fuel enrichment, causing the engine to run at higher rpm.

There are ways of electronically testing the sensors, but I can't find that info sorry - its a check of the voltage across the sensor terminals at set temperatures IIRC.

Unplugging the MAF (not when the engine is running) means that the ECU cannot receive a signal from it, so the ECU uses a default set of parameters held in its tables instead. If the engine runs better with the MAF unplugged, then the MAF must be sending inaccurate signals to the ECU, causing it to miscalculate the fuel needed. If it makes nigh-on no difference, then the MAF signals must be ok, so you can plug it back in (engine off) and assume its fine. I must admit that when a MAF has failed on me, its behaved differently to your experience, but its worth a check as its quick and free.

(For info, when my MAF died on a 940 a few years ago, it started and idled fine, but anything other than a feather movement of the accelerator pedal caused it to stall instantly. Unplugging the MAF allowed the car to be driven reasonably normally, albeit with reduced power).

[360beast]

At 20°C resistance from each pin to the head/alternator should be 2500ohms +/- 250ohms and at 80°c it is 327ohms +/- 37ohms

[volvojj]

Originally Posted by RollingThunder

There are two coolant sensors, one for the temp gauge in the dash and one that the ECU uses. Both are located under the intake manifold near the IAC, hence me wondering if the wiring had been disturbed when the IAC etc had been replaced? The wiring is old and brittle, so any excessive movement of the cables could have cracked the cable strands inside the insulation. As the sensors themselves are hidden under the manifold, I wouldn't expect there to be any physical damage to the connectors. Nevertheless, it is possible that a sensor has failed, or at least degraded. I've just ordered both for my 940 as a preventative measure. They are available still, but are now heritage parts.

If the ECU is receiving incorrect temperature signals, then it will calculate the fuel required by the engine incorrectly. When the engine is cold, the ECU sends more fuel by opening the injectors for longer (equivalent to using a choke on an old carburettor car), and when it is normal/at running temperature, it uses a 'normal' amount of fuel - equivalent to 'choke off'. So if your temp sensor is reporting a cold engine to the ECU, it'll be using a level of fuel enrichment, causing the engine to run at higher rpm.

There are ways of electronically testing the sensors, but I can't find that info sorry - its a check of the voltage across the sensor terminals at set temperatures IIRC.

Unplugging the MAF (not when the engine is running) means that the ECU cannot receive a signal from it, so the ECU uses a default set of parameters held in its tables instead. If the engine runs better with the MAF unplugged, then the MAF must be sending inaccurate signals to the ECU, causing it to miscalculate the fuel needed. If it makes nigh-on no difference, then the MAF signals must be ok, so you can plug it back in (engine off) and assume its fine. I must admit that when a MAF has failed on me, its behaved differently to your experience, but its worth a check as its quick and free.

(For info, when my MAF died on a 940 a few years ago, it started and idled fine, but anything other than a feather movement of the accelerator pedal caused it to stall instantly. Unplugging the MAF allowed the car to be driven reasonably normally, albeit with reduced power).

Cannot thank you enough for all the info and detailed explanations.
Let me replay to your questions:
1. RE your question:"... Both are located under the intake manifold near the IAC, hence me wondering if the wiring had been disturbed when the IAC etc had been replaced?"
- It is possible that they (wiring/coolant temperature sensor) have been disturbed when I have taken the car for a change of two gaskets (when turning left there was knock sound due to worn gaskets). All the other repairs (TPS and IAC change) were done after the problem with the high RPM has already occurred.
- Is it better to change both sensors at once or first try with the one for the Injection system? When the engine warms up, the display of the temperature on the speedometer is stable (at around 55%).

2. RE: "...I've just ordered both for my 940 as a preventative measure."
- May I ask where from have you ordered the sensors (is it possible to share the links)? The sensors that I have found skandix.de, only one of them is Bosch:
> https://www.skandix.de/en/spare-part...black/1037368/
> https://www.skandix.de/en/spare-part...splay/1011404/

Also, may I ask are quality connectors available on internet or are they sold in shops for electric equipment (I could not find on skandix.de / on aliexpress there were some for 2-3$ but I am not sure if they are good quality?). Similar question if its a wire problem?

3. Thank you for the explanation why the problem occurs. I will use the info to explain to the mechanic.

4. RE the MAF I hope that its not the problem but we'll see. If it needs to be changed I will have to look for a used one since Bosch new ones are small fortune on skandix.de

Kind regards
ps. After obtaining the parts and changing them I will let you know how it went.