1989 740 110k miles

Just transported my '89 740 to a second home in Phoenix. It always started flawlessly cold or hot.

NOW, (w/ the car in AZ) when HOT, the throttle must be partially depressed and the engine cranks 2-3 seconds before it starts, and it runs roughly for a second or two before settling down to its usual smooth running.

I've read hundreds of posts, and it seems my most likely cause may be the ECT Sensor mounted under #3 intake runner.

I am 1000mi from the car now, and can only bring limited tools when I next return to where it is.

I downloaded a sensor specifications table (below), but I understand that removing the sensor is almost impossible w/o breaking something unless the intake manifold is first removed.

ONE- Does it sound like the ECT Sensor based on the experience of those here.
TWO- What's the best way to get the old sensor out? Doing a quick search, it seems these are under $20 and have a connector like a fuel injector? If so, maybe I should carry one out if this seems like the most probable fix?

BTW, I very occasionally get a "Check Engine" light at start now (In AZ) that goes out after a couple of seconds, or if the car is re-started. Once running, it runs well w/ the exception that when shifted into neutral from drive, it now stumbles a bit, which it never did before.

Any ideas appreciated. It's frustrating being w/o my full set of tools out there, so what tools would YOU bring if they had to fit in a shaving kit sized bag (I have very basic Metric wrenches/sockets/pliers/screw-drivers there already).

Thanks,
Bob

ECT resistance cold for LH2.4 systems should be around 6k ohms at 32 degrees F (0 deg C), 2300-2700 ohms at 68 degrees F (20 C), and 200 at 212 F (100 C).

However, try checking your ECT wiring: Between pins 13 and 5 at the LH ECU (with sensor DISconnected) resistance should be infinite.

Voltage with the ignition ON and sensor connected, measured between pins 13 and 5, should be:
0 C=around 3 volts +/-.5v
20C=around 2 volts +/- .5v
100C=around .3 volt +/- .1v

 

unplug your ECU, and measure the resistance between pins 13 and 5 at the ECU plug, and it should match those values you list, 6000 ohms at 32F, ~2500 ohms at 68F, ~200 ohms at 212F. I suggest getting the car nice and hot, pulling the connector, measuring it, waiting til the block cools down to ambient, and measure it again. 80F would be somewhere around 1800 ohms, 100F (Az summers...) maybe 1200 ohms, just guessing. its a fairly straight line if you plot it.

 

Yes, I got that-

I was curious if a bad ECT Sensor would cause the problem I describe.

If I were close to the vehicle, I'd go check it now, but the car's 1000mi away and I'm trying to line up the most probable REASON and TOOLS and any HINTS so I could show up prepared.

 

It DOES sound like the ECT. Usually though a bad ECT makes the car much harder to start but that would depend on the conditions I suppose...

Removing it is not that hard: stick your hand in there, unplug it, get the right socket, 19mm?, and with the right size extension it comes out easy, minimal coolant loss....

I have never had much luck measuring those things, I just swap with a "known good sensor" and there we go.

 

Well, HOT it is much harder to start than before (when it would start instantly).

Cold, it's still a champ and pops right off.

The one symptom I don't understand is that when you shift into drive, or into park/neutral, it stumbles (but doesn't stall). I was reading a book on Bosch Fuel Injection and it mentioned a special function for such load changes.

Bob

 

an 89 740 is probably Bosch LH2.4 fuel injection. this uses an idle air control valve thats used when the throttle is at idle (as sensed by a microswitch on the throttle body). the idle air controller modulates how much air is let into the engine to keep a constant 700 rpm idle... the idle switch can get munged so the ECU doesn't know its idling, or the idle air controller can get fouled up, so it can't cycle fast enough to modulate the air, this can cause a bog-down when you abruptly load the engine at idle, like when the AC kicks on, or when you shift into drive.

 

I will check the setting of the throttle switch when I get back out there where the car is. As I remember, you set it to "click" just off the idle stop.

Now, reading the Bentley Bosch F/I book text, it says that the idle air stabilizer (sometimes called an idle air control) bypasses the throttle plate with extra air as needed to keep a pre-set rpm, and opens or closes when the load on the engine changes to keep that rpm. Another thing to check, I guess.

If the idle air stabilizer is involved, that, along w/ the throttle switch and ECT sensor make 3 things to check when I get back to the car. That rpm change with load change also just started happening when the hard-hot-start began, so maybe they are all tied together (since there are few coincidences in life).

I remember many years back shooting carb or brake cleaner through them if they were fouled up.

I suppose it could be electrical as well, with poor connections, or a connector knocked off or not fully on.

This is a good review of the Jetronic LH I really didn't want.
It's been so reliable to date that I could forget about it, until now, at least.

 

if your car is LH2.4 and has the diagnostic box, there's a 'DTM 3' test mode that will cycle the various actuators in sequence, so the fan will go high/low (If you have a 2-speed electric fan), then the IAC will click on/off/on/off a few times, then the injectors will click a few times, and whatever else, then repeat the sequence.

this can be really handy for testing this stuff.

see LH2.4 Diagnostics

 

Hmmm...

I will have to research if my car has this. I've seen nothing resembling a test-receptacle on the back of the driver's strut (I've spent a lot of time there recently replacing my A/C components, inc. the compressor) or anywhere else I have seen (can't look again until I visit the car in AZ).
My car is an '89, and the FAQs say 88-On, so it seems I SHOULD have one, especially since I DO have a Check Engine light in the cluster.

I don't think I have a 2-speed fan. I had 80s BMWs w/ 2 speed fans that had dual temp sensors and a ballast resistor, but my car has only one sensor and I've noticed no such resistor near the fan. My car was manufacturer 12/88, so it SHOULD have the equipment since it said "'88-ON."

Thanks,
Bob
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From the FAQs

Volvo started using self-diagnostics on its Bosch LH 2.4 engine management systems in 1988 on 700 series non-turbo cars and in Regina-equipped cars.

Earlier Bosch LH 2.2 and Turbo cars until 1990 had very limited self-diagnostic capability using an LED tester.

In LH 2.4 cars, If a system fault occurs, then the "check engine" lamp will illuminate, signifying the presence of a fault code. For every model year using LH2.4 up to and including the '95s (except 200 series DLs and GLs), you can flash out fault codes, as well as perform input and output testing, through the OBD-I diagnostic connector unit simply by inserting a little self-contained probe and pressing a button.

You do not need a special scan tool to read codes. As electronic systems were added to more Volvo models, more socket options and even more diagnostic connectors were added.

Unfortunately, you cannot use a generic code reader on any of these OBD-I cars: you have to manually extract the codes from the systems as noted below. 1996+ cars have the entirely different OBD-II system which requires a computerized code scanner to read codes through a special data link.

The diagnostic connector units for OBD-I systems are small black rectangular boxes mounted in front of the left-side(driver-side in LHD countries) shock tower.

Earlier cars have only one unit ("A"); later cars have two ("A" and "B"). Diagnostic connector "A" contains the test terminal probe (the wire mounted on the side of the box in the picture) used in both A and B along with the test button and the LED readout lamp.

 

OBD-1 is nothing like OBD-2 on 1996 and newer cars. the diagnostic box is under the hood near the left side suspension tower and power steering reservoir, little rectangular black plastic box, has a snap on cover, a little jumper wire, 8 holes numbered 1-8, a button and a LED. no code reader is used at all.

 

Yes, I know OBD II is a totally different deal.

In my post above I mentioned I saw nothing like that block on the driver's side strut when I was digging in there the last time and replacing the A/C compressor that's very close to the designated location. My car "should" have it since it was built in 12/88, of course.

I'll check the next time I'm in Phoenix with the car, however.

 

its on the front side of the left strut, probably near the power steering reservoir, and next to the EGR actuator if you have EGR.... little black box, maybe an inch long, with a plastic cover that slips off and exposes the 8 holes, button, and jumper (actually, the jumper is stowed by sticking into a loop on the side of the cover).