240: Troubleshooting Idle Speed (LH 3.1)

[seanile]

1992 240 B230F M47 LH3.1, 5spd manual.

The problem:
Idle speed for my LH 3.1 is supposed to be (with A/C off) 775+/-20rpm. My idle is steady at around 950rpm, no surging, no hesitation.


Things I've done over the past two weeks to troubleshoot:

• I've run diagnostics and there are no errors and no check engine light.
• I've replaced the entire PCV system except for the return hose that goes into the oil pan and the hose that goes to the brass nipple on the cam side of the intake manifold.
• I've removed and cleaned the throttle body.
• I've replaced the throttle body gasket.
• I've removed and cleaned the IAC, confirmed it works via OBD.
• I've replaced the IAC hoses.
• I've removed and cleaned the MAF and the hoses on either side.
• I've replaced the airbox thermostat.
• I've replaced the air filter.
• Replaced the spark plugs.
• The fuel pressure regulator works correctly.
• I've just poured in some liquimoly injection cleaner.
• I've sprayed carb cleaner around the intake area and fuel injectors and there was no surging.
• There's no binding on the throttle cable. There's too much corrosion to adjust the control rod's lock nuts, but it doesn't seem to be a contributing factor.

I am about to go test the throttle sensor resistance with my bud, will update afterwards.
edit: throttle position sensor seems to be bad? Ohm resistance with ignition both off and on was..

• terminals 2 to 3, 3.125k. supposed to be 2800.
• terminals 1 to 2, 2.387k. supposed to be 1200.

wasn't able to get any voltage readings. ordered this replacement: https://www.ipdusa.com/catalog.asp?sku=114719

Left field question/guess: is it possible the A/C microswitch is stuck on causing the idle to sit at the 900 +/- 20 rpm that the car automatically increases to when A/C is engaged?
I've never had the A/C on since the previous owner said it didn't work. The heater valves are also busted so I am forever getting a stream of hot air from the vents.

Looking for suggestions on what to check next.

[bobxyz]

See Bentley section 250-3, or link here, on how to check and adjust the throttle linkage. If your throttle plate isn't closing enough, idle will be high.

To test the LH3.1 TPS, unplug the harness and measure ohms of resistance between the pins of the TPS itself. Ignition on or off doesn't matter since it's unplugged. If you rotate the throttle cable cam by hand, the pin 1-2 measurement should go up and down. Your 3.1K (3100ohms) measurement is a bit high, but alright by itself. Your 2.387K (2387ohms) measurement is pretty far off.

Both A/C and shifting out of park/neutral on an automatic can increase the idle speed. If you can find a LH3.1 wiring diagram (maybe from a 740/940?), you can probably find the A/C and P/N wires on the ECU page. The P/N interlock, or jumper for a manual, are on the 240 Starting Circuit page.

[2manyturbos]

Originally Posted by seanile

1992 240 B230F M47 LH3.1, 5spd manual.

The problem:
Idle speed for my LH 3.1 is supposed to be (with A/C off) 775+/-20rpm. My idle is steady at around 950rpm, no surging, no hesitation.


Things I've done over the past two weeks to troubleshoot:

• I've run diagnostics and there are no errors and no check engine light.
• I've replaced the entire PCV system except for the return hose that goes into the oil pan and the hose that goes to the brass nipple on the cam side of the intake manifold.
• I've removed and cleaned the throttle body.
• I've replaced the throttle body gasket.
• I've removed and cleaned the IAC, confirmed it works via OBD.
• I've replaced the IAC hoses.
• I've removed and cleaned the MAF and the hoses on either side.
• I've replaced the airbox thermostat.
• I've replaced the air filter.
• Replaced the spark plugs.
• The fuel pressure regulator works correctly.
• I've just poured in some liquimoly injection cleaner.
• I've sprayed carb cleaner around the intake area and fuel injectors and there was no surging.
• There's no binding on the throttle cable. There's too much corrosion to adjust the control rod's lock nuts, but it doesn't seem to be a contributing factor.

I am about to go test the throttle sensor resistance with my bud, will update afterwards.
edit: throttle position sensor seems to be bad? Ohm resistance with ignition both off and on was..

• terminals 2 to 3, 3.125k. supposed to be 2800.
• terminals 1 to 2, 2.387k. supposed to be 1200.

wasn't able to get any voltage readings. ordered this replacement: https://www.ipdusa.com/catalog.asp?sku=114719

Left field question/guess: is it possible the A/C microswitch is stuck on causing the idle to sit at the 900 +/- 20 rpm that the car automatically increases to when A/C is engaged?
I've never had the A/C on since the previous owner said it didn't work. The heater valves are also busted so I am forever getting a stream of hot air from the vents.

Looking for suggestions on what to check next.

What to check next? Your actual idle speed. The tachometers in a 2/7/9 series vary quite a bit in their reading.

[84B23F]

Originally Posted by bobxyz

If you can find a LH3.1 wiring diagram ....

Best I could find here, and I've looked for years for one. Two computer boxes, on left top pic LH 3.1, and right bottom EZK. I'd save this pic OP.

Throttle Body Adjustment here pic

>A/C microswitch

Looks like pins 14 and 15 are related to AC on LH box

At AC dryer in engine compartment, one could check for voltage, with car running...just remove plug, insert voltmeter on removed plug side

[84B23F]

Footnote - LH 2.4 pinout and LH 3.1 pinout may or may not be the same...I don't have the time to compare, but here is LH 2.4's here, and LH 3.1 here

[seanile]

Originally Posted by bobxyz

See Bentley section 250-3, or link here, on how to check and adjust the throttle linkage. If your throttle plate isn't closing enough, idle will be high.

To test the LH3.1 TPS, unplug the harness and measure ohms of resistance between the pins of the TPS itself. Ignition on or off doesn't matter since it's unplugged. If you rotate the throttle cable cam by hand, the pin 1-2 measurement should go up and down. Your 3.1K (3100ohms) measurement is a bit high, but alright by itself. Your 2.387K (2387ohms) measurement is pretty far off.

Both A/C and shifting out of park/neutral on an automatic can increase the idle speed. If you can find a LH3.1 wiring diagram (maybe from a 740/940?), you can probably find the A/C and P/N wires on the ECU page. The P/N interlock, or jumper for a manual, are on the 240 Starting Circuit page.

ok great, thank you. the linkage is a bit too frozen/seized to adjust it and i cannot find a source for a new one at the moment. the ohms being pretty off and the lack of voltage readings brought me to order a new TPS (IPD has a Bosch OE one for $35, hadn't realized it'd be that affordable) so we'll see if that helps.

Originally Posted by 2manyturbos

What to check next? Your actual idle speed. The tachometers in a 2/7/9 series vary quite a bit in their reading.

this is not something i'd considered actually. any guidance on how to test the stock one, or am i limited to getting a cheap redundant tach?

Originally Posted by 84B23F

Best I could find here, and I've looked for years for one. Two computer boxes, on left top pic LH 3.1, and right bottom EZK. I'd save this pic OP.

Throttle Body Adjustment here pic

>A/C microswitch

Looks like pins 14 and 15 are related to AC on LH box

At AC dryer in engine compartment, one could check for voltage, with car running...just remove plug, insert voltmeter on removed plug side

awesome, thanks for the adjustment link, i'll check that tomorrow. from what i saw with the butterfly valve while i was cleaning it today, it shuts pretty tightly/fully.
i'll also look for voltage at the A/C dryer tomorrow, thank you.

i PMed you a link to a 1992 wiring diagram set, page 21 has the wiring diagram for the LH 3.1, http://brickspeed.net/VolvoDocs/240-..._240__1992.pdf

[2manyturbos]

Oscilloscope, tach/dwell meter etc. There is no way I would ever use a Volvo tachometer as a diagnostic tool. That is your first mistake.

[seanile]

i fine tuned the throttle cable tension, control rod, and idle & WOT pulley stop positions per bentley 250-3, and checked the butterfly position as well using the above link from 84B23F. then i installed a new TPS and it doesn't seem like anything is different. lost daylight before i could check the TPS electrics and still have to check the a/c's circuitry for any sneaky voltage.

i'll check the car's rpms at particular speeds to see if the tach reflects reality at the appropriate gear ratios. also going to double check the fuel pressure regulator, a friend said that the vacuum should hold 3 bar steady.

would love any other suggestions!

[84B23F]

Originally Posted by seanile

TPS electrics

Never messed with it, nor read specifics, but its been suggested it has two functions:

1. WOT - Increases fuel enrichment
2. Idle RPM Only

Idle air control actuator (IAC actuator) was designed to work at idle-RPMs only, not when driving with engine above idle RPMs. So, TPS must be adjusted to spec...foot off throttle pedal, switch's circuit for idle side is working.

A brake booster can increase idle RPM when it starts to fail...so if its hose is removed from intake manifold, and the intake port plugged, this can be checked. Any air leak into intake side will increase idle RPM.

Do note when reading, LH 2.2 is not the same as LH 2.4. Here is a FAQ, but examine this PIC closely, and notice what it says when current is off (aka unplugged IAC).

FPR - Most likely a waste of time since O2 sensor via ECU can compensate.

AMM/MAF - If idle is erratic, sometimes this is the issue. From AMM to throttle body, that "hose" can't have any holes in it...all intake air must pass thru AMM. AMM has a "burn" cycle to clean its sensing wire...AMM/MAF cleaning spray is expensive, but AMM could be sprayed...but this action may do nothing in regards to idle issue.

[seanile]

The LH3.1 that i have doesnt have a switch unfortunately.

Have already cleaned the MAF and the hose connecting from it to the TB. Didnt see any cracks.

I'm unfamiliar with the brake booster though will do some reading.

[84B23F]

Originally Posted by seanile

The LH3.1 that i have doesnt have a switch unfortunately.

RE: "installed a new TPS"

I think I lookuped part number, and this "switch" is applicable to LH 2.4/LH 2.2 and LH 2.1, iirc.

This switch has two functions:

1. WOT - Increases fuel enrichment
2. Idle RPM Only

and at ECU, I suspect there are two pins that could be touched to "see" if either of these circuits are active, or not active. Same with AC's pin(s) at ECU. I'd use a high impedance volt-meter, or high impedance test light hooked up to battery power to evaluate at ECU.

[seanile]

Originally Posted by 84B23F

RE: "installed a new TPS"

I think I lookuped part number, and this "switch" is applicable to LH 2.4/LH 2.2 and LH 2.1, iirc.

This switch has two functions:

1. WOT - Increases fuel enrichment
2. Idle RPM Only

and at ECU, I suspect there are two pins that could be touched to "see" if either of these circuits are active, or not active. Same with AC's pin(s) at ECU. I'd use a high impedance volt-meter, or high impedance test light hooked up to battery power to evaluate at ECU.

i think we're talking past each other. my system's (the LH 3.1) throttle body has a sensor that isn't limited in function like the LH 2.2's and 2.4's switch, it's a potentiometer. see: https://www.brickboard.com/RWD/index...613&show_all=1

[84B23F]

Originally Posted by seanile

it's a potentiometer

Ok, two basic differences between 2.4 and 3.1...OP there got the CSV wrong...if a LH 2.4 vehicle had one, it most likely was an earlier model. My 1989 has one, but not the 1993.

RE: "predictive" fuel management & better fuel economy

That's BS....fuel economy is based upon driver's behavior...I can still get close to 30 mpg with auto transmissions....3.1 were installed in stick shifts, iirc.

Ok...up to speed now....

[84B23F]

With engine warmed up, was the electrical plug removed from IAC, and then engine re-started? Idle did what?

Logic...at idle, several sensor devices (TPS, neutral switch...) could be used to inform ECM/ECU an idle state exists. But, AC engagement electrically would skew this state. So, if the mechanical aspects are near OEM design, then its an electrical/ECU issue.