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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