My 89 740 GL saga: J Sweet

Unplug the AMM and see if it runs any better.

I had an 89 740 that exhibited similar behavior and it was the wiring harness. The wiring was shorting out right at the connector by the passenger side headlight. I forget what it is called but there is a big square connector over by the passenger headlight. I know that electrical problems usually cause the car to buck and fuel problems casue the car to die slowly. Mine was electrical that was shutting off the gas flow and starving the engine of fuel.

Regards Russ

Fuel pressure regulators have been known to go bad - a quick (and not positive) test is to pull the vacuum hose off the regulator and see if there is gas inside the hose. If so, you can be sure the regulator is bad. If not, you still don't know for sure.

Mike

That makes me really wonder about the fuel pressure. The O2 sensor can't put the mixture off that much, and the limp-home setting isn't right either. There isn't much left besides fuel pressure being too high. Time to get serious about measuring that.

I assume *all* the plugs are sooted up?

Mike

Is there a junkyard near you? I've always just kept a spare around to swap in if there's a problem, it's easier than trying to test the pressure.

I cut the fuel supply hose about 1 1/2 inches from the fuel rail fitting and inserted the tee there, then removed the short piece and connected the hose back to the fitting when I was through. With the vacuum hose removed from the fuel pressure regulator (and the hose plugged) the corect reading is 36 psi, or 42 psi for the turbo model. Reconnecting the hose should drop the reading by the amount of manifold "vacuum" - about 10 psi less fuel pressure at idle. What you are looking for is a big deviation, like double the fuel pressure.

In the '89 I don't know if the check engine light would come on with the rich mixture, but it would make sense that it would. Just a wild thought - when the battery failed in my Nissan 300ZX the mixture went way rich (so the top speed was about 40 mph and there was a black cloud following me) because the ECU was constantly being reset by the noise from the alternator; no check engine light. Oddly, the engine still cranked okay but replacing the battery fixed the problem. If you have a DVM around a check for AC voltage across the battery terminals would ward off a few evil spirits - it should be below 0.1 VAC. In addition, there should be less than 0.1 volt DC between the engine block and the chassis - more than that means the engine ground strap is bad.

Mike

Autozone doesn't have a fuel pressure tester, so I am shopping for one on eBay. This one is available (at a very good price),

it has an adapter which actually is made to fit GM, but it is metric. F itting size: M16 x 1.5 I don't have a clue what the fuel rail fitting size is on my 740, so can't tell whether this one will work. Then I have the problem of extending the gauge hose, as 3 ft isn't going to get the gauge located inside the car to be read while driving. I think it is best to buy a guage rather than shotogun a new fuel press regulator, as my brother has a 740 GL also, and I am probably going to buy another one myself. I haven't had good experiences with parts from salvage yards, so I would have to pay $52 for a new regulator, posibly only to find it wasn't needed.

I gather that since fuel consumption has gone way up, then the fuel rail presure might be too high at low speed or idle, and then way too low when punching the accelerator? I checked the oil for signs of gas in it...it is okay. I pulled the vacuum line off of the fuel press. regulalor while running (immediately closing off the open end of hose), and there did seem to be a slight difference in the sound of the engine (possibly indicationg a change in fuel rail pressure/fuel delivery)....but it was very slight.

I checked the spark plugs, and they have a light coating of blackish stuff, but not jet-black soot...and there is not any smoke coming from the tailpipe. They all looked identical.

Thanks, Jim

Since it's got a malfunction of that magnitude but no codes are being set, I'd suspect the ECU itself. I had a similar problem with no codes and it was the computer itself, hence no codes. I would, however, test the fuel pressure/fuel system.

I figured it out!!! That 3 in. black plastic duct running between the AMM and dthe intake manifold was the problem. It has gotten very weak, flexible, and so even though the air filter didn't look very dirty at all, there was enough of a vacuum in the duct when you punched the accelerator that the duct was collapsing right near the intake end. And it was collapsing on the BOTTOM side so couldn't be seen readily. The duct finally "popped" inward and stayed that way after shutdown, so I spotted it. Now the problem is getting a replacement. Autozone doesn't have one, and there isn't a Volvo dealer where I am. A generic duct won't work because the AMM end is about 3 in., and the intake end is about 3.5 in. ID. So all I could do for now is wrap it with metal mending tape to help it hold a circular shape. Regards, Jim

Isn't there a junkyard nearby you could get one from? The tape should hold up for now, you might also be able to mail order one.