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I'm struggling with high idle on a 200k mile 1986 240 with LH 2.2 and AW70. The idle is high in cold and hot conditions, probably close to 2500 RPM.
I started by smoke testing the intake side but didn't find any obvious vacuum leaks. I disconnected the charcoal canister circuit anyway, just to be safe. Then by pinching the intake side of the IAC plumbing I was able to get the idle down to something like 500 RPM based on ear (no tach) when the IAC was removed from the air intake circuit. Similarly, grounding one of the pins on the diagnostic port on the driver's side (which disables the IAC) would also drop the idle to very low levels.
So, thinking I found the culprit (a stuck open IAC), I measured resistances between pins and found thousands of ohms between the pins. A normal IAC is around 20 ohms between 2 of the 3 pin combinations, and 40 ohms on the 3rd combination. So I disassembled the IAC as the LH 2.2 units are rebuildable. The internals were actually very clean and not gummed up, however, I found giant gouges in the stator surfaces and extremely worn and oxidized armatures from arcing. This seemed consistent with the high resistance values. I then cleaned away the oxidation and reassembled the unit, which now measured the correct 20 and 40 ohm values. The unit sort of worked after that, though I would need to knock it with a screwdriver occasionally to get the idle to move into the correct speed. I attributed this to the armatures getting "stuck" in the deep grooves they wore over 200k miles.
At this point, it seemed the best option was to replace the IAC. Bosch units are NLA, except for used unknown units on eBay, so I bought one of the fairly common aftermarket IAC's from IPD. Upon installing the new unit, the idle remains high in almost the same way. Unplugging the battery to force the ECU to relearn has not helped.
If I blip the throttle enough to disengage the TPS, the idle will drop to a normal speed for about 1 second and then slowly build back to up around 2500 RPM over the course of about 5 seconds, almost like the IAC is slowly opening all the way up. I can get the idle to drop to normal levels if I ground the diagnostic pin or pinch the hose, same as the first IAC.
I'm looking for ideas on what my next steps should be. It seems like the IAC circuit is working, but something is driving the ECU to open the IAC all the way open. The car drives normally otherwise, and gets what I consider decent mileage (around 16 mpg around town). No other indicators of issues that I can notice. I've previously done most of the stage zero items (plugs, wires, air filter, distributor, PVC clean, fluid changes, etc).
Any thoughts as to what I should tackle next?
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As a follow up to this issue, and a thank you to all the insights, I did end up finding the problem. It was the brand new replacement IAC from IPD. They sell an aftermarket unit which definitely didn't work. Sourcing a used Bosch IAC fixed the problem instantly.
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For excessive high idle problems that change with temperature like that, I would have two main suspects a) missing signal from the microswitch on the throttle shaft (ECU doesn't even know it's supposed to be maintaining idle) and b) bad signal from the ECT (either bad wiring connection, or failed temp sensor, rearmost of the two under the intake manifold). Inspect wiring carefully, especially as yours is square in the disintegrating wiring insulation years. Then time to break out a multimeter. There are specs you can find for the temp sensor.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Thanks Dave. I haven't dug into the green books yet, but do you know if you can simply check continuity from the temp sensor and throttle microswitch back to a pin on the ECU plug?
I had previously discounted the temp sensor thinking that failure there keeps the car in ultra rich open loop mode, which results in terrible gas mileage, something I don't think I'm experiencing.
Also, I've been thinking about the MAF. In past cars, MAF failure has resulted in pig rich running as the system abandons closed loop control and defaults to a safe fuel map. However, the LH 2.2 MAF has an adjustment screw for idle mixture, and some procedure for setting this in the green books. Could failure or misadjustment of the MAF do something to force the ECU to drive the IAC all the way open?
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Hi,
You asked for pin out for a LH 2.4.
Pin two is for the throttle switch and pin 13 is for the coolant sensor.
I think Art Benstein has put out, in one of his posts, a chart that has voltage readings for pin 13 that tells the temperature @ certain voltage. He has amazing talents.
We discussed the idea of having a permanent voltmeter on that circuit to monitor the going on’s down there.
Another one of my foolish ideas until something like this happens but it could be very wide ranging thermometer.
They put thermometers as a cabin accessory options for luxury folks, so, they will know if they are hot or not. 🤭
The thoughts were left off as it would have to be a digital display one and not load the circuit in anyway. An analog voltmeter might add some resistance. The sensor operates on that principle.
Dave is right troubleshooting from above as it does reduce damaging a whole bunch of pins inside that ECU connector. Not that I have had any issues because I prefer back probing from the income wire side on sockets and more so on everything delicate.
The ECU was purposely made to make things more tamper resistant to get reliability.
Phil
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Hi Phil. His 1986 240 B230F is LH 2.2 not LH 2.4 (1989-on). The ECTs for these two systems are different, but have a similar/same spec. This is noted in the 700/900 FAQ article I referenced.
The LH 2.2 ECT is Volvo p/n 1332396 (typically a blue connector) and is a single thermistor. One connector pin goes to chassis ground (black wire to the manifold) and the other to the ECU (pin 2, typically a blue wire for 240s and a grey wire for 700s). At the LH 2.2 ECU connector, the main ground for meter reference is pin 5 (to the fuel rail ground screw on the intake manifold).
The LH 2.4 ECT is Volvo p/n 1362645 (typically a black connector) and is a compound temp sensor with two thermistors, each on one of the two pins. The sensor case is the ground to the block. One thermocouple half is for the ECU (connector pin 13, typically a grey-white wire) and the other half is for the EZ-K ignition controller (pin 2, typically a red-black wire). At the LH 2.4 connector, the main ground for meter reference is pin 17 (to the fuel rail ground screw), while pin 5 is a separate chassis ground (for the O2 sensor shield).
Without doing more research to clarify and verify what's in the FAQ, it appears the resistance specs for various temperatures for the two different ECTs are the same, but the LH voltages in the circuit during operation may be different.
Basic testing can be done at the ECT if you can reach under the intake manifold and successfully get the connector off to get an alligator clip wire on the pins. When testing the LH 2.2 sensor you need to measure across the two pins, which may be a bit fiddly to do, so perhaps easier to do at the ECU connector. When testing the LH 2.4 sensor you measure each pin separately in reference to a chassis ground (like the fuel rail ground screw). If you were to measure the LH 2.4 sensor across the pins you would get the combined resistance, so double the spec.
For more accurate measurement comparing to the spec and allow for wiring/connector resistance you need to do it at the ECU connector, requiring side panel removal for access and removing the connector shell. For the LH 2.4 ECT you need to also test at the ignition controller connector, requiring under dash removal for access, which is a bit more of a nuisance.
If Art's following this maybe he can chime in here with any additional notes and specs.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Hi Dave ,
You are absolutely right his is a 2.2. I didn’t remember that.
I have only one of them but three 2.4 cars. They are built off the early LH platforms.
I have two cars. One is a CIS 78 and my 84 is the first of the LH series.
IT might be a 1.0. It has a AMM unique to itself.
I think it can be rewired to accommodate a 007 don’t know about a 016.
The early years all had their ignition boxes were under the hoods. Until the EKZ.
The ECU cases stayed on the passengers side foot well areas of the vehicle throughout productions.
They seemed to stick to things they like and worked for years!
What I seem to have a problem with is the dashes. They all look so similar I have to stop for a moment to try to remember what maintenance I did, to which one. The 1978, being my first Volvo started me down a long path of used vehicles. Never bought another new vehicle, just appreciated the upgrades as they came.
Excuse me for rattling and being confused on the LH’s.
You pack in so much information, it helps get me straight again.
Thanks
Phil
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Initial diagnosis of both the throttle microswitch and the ECT is best done under the hood, first a visual inspection then with a meter. Later you can verify the signals at the ECU connector pins. (FYI. if and when you ever need to, be sure to probe into the side of the connector, not into the pins as the tiny fingers are easily bent and when you force the connector on may short and/or suffer permanent damage.)
You need to stay focused on the basics here. Begin by listening/feeling for the microswitch click as the throttle opens and closes. You can verify operation with a meter, but they rarely fail. If you've been playing with the throttle body stop screw (which you shouldn't have needed to) then the switch may well need readjustment. The wiring from there to the ECU is normally okay unless you've been playing with connectors at the firewall or ECU.
Following that, a bad ECT is well known for causing high idle issues. ECT's generally fail to open and you can end up with a wildly high idle of 1300 rpm or even higher. You can verify basic ECT functionality measuring resistance across the pins (using alligator clip test leads, be sure to keep the tips separated). For a basic functionality check, you don't want to see more than about 3K-7K Ohms with a cold engine. You can find the actual specs here in the 700/900 FAQ (read the whole ECT section for a full discussion).
https://www.brickboard.com/FAQ/700-900/EngineSensors.htm#DiagnosingECTFailures
When you start getting an idle much over 1500 rpm then I start getting concerned the ECU connctor wiring is no longer intact. Wiring under the intake manifold is especially sensitive to crumbling insulation in the mid-1980s 240 models. A short inside the harness is even possible. In that case things may get worse when you start handling the ECT cable. You will be able to confirm that when it comes time to check at the ECU connector. Also, if I recall correctly, with effort it's even possible to have the two temp sensor connectors switched. I have a funny feeling if you were to disconnect the ECT and try running it would be similar to what you have now.
Once you've got the ECT verified as okay then you go back to basic idle setup, starting with a good TB cleaning on the bench, checking the throttle plate stop screw at the same time, checking IAC functionality and using the thumbscrew to adjust base idle with the IAC test connector lead grounded to keep the IAC closed.
Interesting that you took the IAC apart. They're not considered serviceable and generally shouldn't be pried open, even for cleaning. Apart from grunge at the bushings affecting IAC operation, the armature wear grooves you found can be verified by tapping on the IAC during idle and noting any idle change. I once tried dressing out the ends of the grooves in the armature, but they're usually too deep to get any kind of lasting fix. You can try getting a bit more life out of them by opening the thumbscrew a bit to raise base idle into the 800-850 rpm range. The contacts then won't spend as much time at the step at the end of the grooves.
You can forget both the AMM and O2 sensor for now. You've got a basic idle control problem, not poor accelaration, engine rough running and fuel mileage. You will normally never need to touch the AMM adjustment -so don't even be tempted.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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I there anything here that could help with your problem?
https://www.brickboard.com/RWD/volvo/1213870/220/240/260/280/idle_adjustment.html
Your mpg is bad but probably due to the engine revs rarely being below 2500 rpm.
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As far as I can tell, I'm actually able to complete the idle adjustment steps that Dave outlines. Once the IAC is disabled, I can use the set screw to get a nice low idle. It's only when the IAC is reengaged that the idle runs away.
My low idle with a disabled IAC plus the results of my smoke testing makes me believe it isn't a matter of chasing vacuum leaks at this point.
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