I’ve been chasing a low MPG problem for months now. See here for lengthy thread on it: http://www.brickboard.com/RWD/volvo/1582312/740/760/780/mpg_decrease_injectors_wiring_harness.html.

Symptoms: Car runs rich, can be smelled at idle even after warm-up. Idle also erratic but intermittently erratic. Sometimes idle steady, then it creeps up 200 rpms. Sometimes just slowly moves up and down 200-300 rpms.
Barely getting 15.5 MPG in city driving! Ugh, the ugly side of Volvos…

Otherwise, car runs like a champ. Excellent acceleration, starts fine, etc.

Work: So I diagnosed the wiring more. Checked ECT connector to ECU, checked ECT circuit to AMM, checked oxygen sensor signal to ECU, o2 heater wire and ground. My car is one of the bio-degradable wiring years. Luckily, my car has not suffered unusual rot.

I did find test connector 220 red-white (rw) wire wasn’t open, but had a 6 kohm resistance. This odd because it leads straight to ECU. My ECU is reman from Fuel Injection Labs, it could be off but these guys are known for top notch remans.

On the second weekend of testing wires, I unwrapped the wiring group as it comes out of firewall on passenger side. Theorizing there was rotted wiring in there, hence the resistance in 220 circuit. Nope, wiring is healthly looking, only some rot at A connector that is exposed.

Then I retested the O2 sensor circuits because the heater wire didn’t map the way I thought it should compared to wiring diagram. Found that heater wire had resistance when grounded, before I was testing continuity from end to end of wire. Checked wiring, guess where that originates? Yup, fuel injection pump relay, that in 1986 year is called Jetronic fuel system relay. This is before the radio suppression relay years, my car doesn’t have one. Relay part no is 3523608, Stribel no 898151.

Looking at the wiring diagram this relay connects AMM, IAC, injector ballast, tank pump, main pump, ground signal to/from ECU, charge pressure sensor for turbo. So a flaky relay is upsetting a lot of electrical signals. The old relay was dated January 1997, so it’s been replaced before!

New relay only installed last night. Idle much more steady! Gas pedal very sensitive. Exhaust did not smell as rich as it has. We’ll have to wait to see if MPG improves… crossing my fingers!

I didn’t have time to retest heater wire, and test-point 220 rw idle ground control. I am curious what I will find.

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Checked out my spark plugs for being dirty/fouled from running rich for the past year. When I got to no. 3 it was finger loose! Tip looked decent light brown color. This was my own poor tightening work.

Yes, indeed my MPG is back to 17 city 23.5 highway, for LH 2.2 turbo that's good people.

So between the intake manifold gasket leak and spark plug air leak was the problem. Everything else wasn't likely necessary. Dang these vacuum leaks!

By the way, the spark plugs are Denso iridium power plugs, and they run fantastic in my car.
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Replaced intake manifold gasket over the weekend. So far it is looking like an improvement in MPG with idle more steady. Creeping tach seems gone, just jittery.

Job was easy, except for removing glued on gasket! This was a Volvo shop that did this. Took me 2 hours to completely remove small pieces around bolts.
That glue probably reduced the chance the gasket could smash out flat under pressure from manifold, over time the unglued parts created leaks. All of the gaskets bottoms, toward ground, had oil stains on them (hard to spray something underneath the manifold to test for leaks).

Also replaced the intake hose from AMM to turbo the week before. Old one was collapsing, not on main hose, but the blow off valve outlet. The weight of the BOV was pushing down on a oil weakened hose. I think was reducing the effectiveness of the BOV operation. I had a NOS one that I got last summer.

I'll post MPG after next fill. This fill was 17.2 mpg which is good for this car!
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After more electrical diagnosis work, I discovered the 6k resistance found in test point 220 leads to .... ignition box terminal 15!
Going backwards, the "circuit" goes test point in engine bay to ECU term 12 as R-W wire. But there is also a R-SB/Black wire starting at ECU connector going to ICU terminal 15. Previously, I was ignoring this R-SB wire, d'oh.

Tested continuity of wire with connectors dis-connected, both R-W and R-SB. Then re-connected ICU connector and back probed term 15 to ground, voila! Found the 6k. I've got an ICU box on order.

Oh, and before above I swapped 2 other ECUs in to see if test point resistance changed. The only thing I discovered here was the reman ECU has reversed resistance readings from the 219 and 220 test points (I wonder if that affects anything?)

I also did some continuity testing of TPS circuit and it is mostly good, it has 0.6 ohm of resistance which is higher than ideal, usually I get .1-3 ohm continuity readings without zeroing out my DVOM.

I can't tell you how happy I will be if ICU fixes my MPG!!
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In my continuing saga of how not to fix a car:

Replaced ICU. MPG remains the same after both city and highway driving. Boy, my methods are wack - LOL!

I did figure out the 0.6ohm measurement. The Green Book says both 219 and 220 should read OL when ECU harness is disconnected. But 220 is connected to ICU and ECU (the R-SB wire), so of course, it won't be infinite! When I disconnected the ICU and ECU harness, 220 was OL. So all is good there! Red herring from Green Book!

Now considering leaking intake manifold. I was able to tighten about half of the nuts on it a 1/4 turn. Idle a tad more steady. I had a head gasket replacement in April 2012. Checked exhaust manifold nuts too, but they were tight except one nut on no. 4.

I will spray manifold this weekend to see if idle is affected. Carb cleaner/ WD-40 seems like flammable no-no, but it is often recommended. Alternatives to this, like soapy water work, as I have read, work??

Car idle descripton after cruising on highway for over an hour: putt-putt-putt-putt-phitt (repeat) Something is still not right.

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Propane is safe. I wouldn't use soapy water since if you do have an intake leak, you're sucking it in.
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Post Back. That's whats makes this forum work.

Okay, did more electrical testing:

the 220 test point reads OL like it should when DVOM is grounded to battery. When test lead grounded to manifold I get the 6kohm resistance. That seems odd? So I've ruled out a bad wire there for now. But still don't understand why this is so, maybe will need to revisit.

O2 heater wire, well, it should have some resistance I believe.

Anyway, reading around here I found a comment by porkface that running rich, might be, a dying coil that isn't firing the spark enough to burn the fuel. Tested my coil, it's on the edge of specifications. So decided to buy a new one considering it's likely the original.

Curious, if anyone thinks the weak coil could cause rich condition? cause unpredictable idle, sometimes steady, sometimes creeps, sometimes engine chugs.


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Forgot to mention I cleaned the IAC with Lectra-Motive by CRC. Some brown liquid came out but I wasn't expecting much because it was new in the last two years or so. (found a sweet NOS one on ebay for great price)

Also, cut off old connector to IAC and replaced it with one from Napa, part no. EC173. Pricey at $15.

Results: Idle a tad smoother, but still have creeping rpms up or down, and chugging engine. T

hese symptoms could be bad coil? Would fit unpredictablity of idle in that spark is uneven?
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After a couple weeks, the idle is back to flucuating, drat.I'm going to redo IAC connector and gently clean IAC. I wonder if this change MPG?

Also MPG is still off 2-3 mpg both city and highway.ugh. I'm getting old pick-up truck MPG! 14-15, and 20 at 65 mph! Just sucks, considering everything else is working great. I wish cash for clunkers would return...

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Hello,
disconnect the negative terminal from the battery. Using a test probe or test light, touch the cable with the test wire. Touch the negative terminal at the same time with the probe. The light should shine brightly and then fade. If the light remains on, you have a current leak somewhere. You can get a volt meter and test all the circuits one by one in the fuse panel. That should lead you to the source.
Also, the alternator should charge more than 13 volts. More like 14.0 -14.5 or so.
I had my battery go out (new battery) a few weeks ago. Had to have the car towed home. Next day while checking my connections I discovered that the ground wire to the engine block from the alternator was loose. If I had known at the time, I could have finger tightened it and it would have been sufficient to charge the battery. As it was, being dark, it didn't occur to me to check the connections when the battery went dead. So, there ya go... mechanics 101.
Art C.
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89-740 GLE;91-940 Sedan ;98-V90 Wagon ___ all running well

Hey Dan,
you replied to my post about the erratic , high idle speed on my '89 740 GLE. I did a little more research, using your comment as a impetus, and it turns out that even though there may not be an OBD code originating from the ECU re:the throttle sensor, the sensor can still be non-operational. So I jumped the connections: #3 to #18 and the idle instantly decreased. Now when I say that, I mean the idle decreased about 100RPM. It's still high (about 950- 1050) but I have yet to install a replacement TPS I got yesterday at PNP.

I'll post back when I get the TPS re-installed. Oh yeah, when the idle decreased there was a noticeable miss in the firing. I have a suspicion that an injector is not firing as it should. Also the dash light for the 02 sensor comes on (has been for several months now). I replace the sensor, but the light still comes on, and there is a rich smell to the exhaust. What do you think? Faulty sensor again? Can the CAT be burned?
Thanks for the input.
Art C.
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89-740 GLE;91-940 Sedan ;98-V90 Wagon ___ all running well

Hey Dan,
you replied to my post about the erratic , high idle speed on my '89 740 GLE. I did a little more research, using your comment as a impetus, and it turns out that even though there may not be an OBD code originating from the ECU re:the throttle sensor, the sensor can still be non-operational. So I jumped the connections: #3 to #18 and the idle instantly decreased. Now when I say that, I mean the idle decreased about 100RPM. It's still high (about 950- 1050) but I have yet to install a replacement TPS I got yesterday at PNP.

I'll post back when I get the TPS re-installed. Oh yeah, when the idle decreased there was a noticeable miss in the firing. I have a suspicion that an injector is not firing as it should. Also the dash light for the 02 sensor comes on (has been for several months now). I replace the sensor, but the light still comes on, and there is a rich smell to the exhaust. What do you think? Faulty sensor again? Can the CAT be burned?
Thanks for the input.
Art C.
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89-740 GLE;91-940 Sedan ;98-V90 Wagon ___ all running well