I recently purchased a 1986 740. It runs a little rough and the gas mileage is poor (about 100 miles per tank). I suspect a faulty sensor or something of that nature but I don't know where to start. Please Help!!!

I had almost the exact same symptom, I checked everything but the O2 as I didn't know how to do it, well thats what it was.

All the other stuff said has caused others problems in the past too, so it is good advice. The fouled spark plugs are a symptom though, not the cause.
--
89, Gold 745 GL Wagon, 210k known miles. Just a beginner

I suspect you'd have a black sooty layer covering inside the exhaust. It must be running rich to only get 100 miles per tank.

Has the O2 sensor ever been replaced. I don't know if a complete O2 sensor would give you that poor of mileage, but if it's not working it would not feed the rich condition back into the ECU.

Something else might be happening to cause the injectors to dump in excess fuel ... maybe FPR as one of the posters suggested.

Anyone else have wisdom?
--
Norm Cook Vancouver BC; 1989 745T 204,000KM

Hey,
Just start reading. Everything you could possibly need to know is in here somewhere.I'm constantly amazed at the depth of knowledge on this site. The FAQ section is excellent. If you don't see your problem, wait! It'll show up in a day or 2. Most maintenance related fixes are relatively inexpensive and easy to do. My son's 740 GLE was pretty sad when he got it in October, but slowly it has become a sweet first ride for a 16 year old.
Good luck.
Dave

I also have a 1986 740 GLE. A Sweet ride indeed. Too bad it's been laid up for the last 5 years with some serious mis-fire engine sensor issues. I'e been grabbing parts off it now and then for my later 740 and now my 940.

Anyway. 1st thing I'd check is your sparks. A master volvo mechanic chidded me for using cheap sparks and he blames bad mileage and poor running on that. He says the volvo sparks (and champions) are the only ones worth using and that they come for as little a $10.00 a pack.

Also check your Fuel Pressure Regulator (see the FAQs). I had one go bad and it dumped way too much fuel into the injectors - ran rough, fuel pump buzzed under the car, eventually died on me. Easy to check though. With the car running pull the vacuum hose of the fuel pressure regulator (valve looking thing bolted to the right-front side of the engine) If you find gas in the hose, then it's going bad.

From the responses I got here last year concerning my 740 mis-fires and horrible running (when I start it) throtle-body sensor and temp sensor were mentioned. If the car thinks it's really cold, the ECU dumps A LOT of extra fuel into the system. It could be one of many things or a combination of a few, the FAQ is a great place to start (expensive if you want to print it all out, however).

I haven't bothered to trace and fix my old 740. Too little time, or money, or both. My 940 is holding my attention right now.

Good Luck!
--
Volvo #6: all wagons.... 1972 145S, 1976 245 DL, 1983 245 GL, 1986 740 GLE, 1990 740 GL, now a 1995 940...What's Next?

NGK V-POWER (Discount Auto Parts) are excellent spark plugs. They are much more efficient than Champion. I would even prefer them over Bosch. Their special design is different from many of the other plugs in their class. www.ngk.com
1989 740 gle wagon (non-turbo) aw-70 A.T. Currently 390,000 miles
1989 740 same currently 299,000 miles
1988 740 same currently 430,000 miles
avg hwy gas mileage 26-30 mpg, about 380-425 miles per tank
use super unleaded, all synthetic lubricants, and Lucas fuel treatment (upper cylinder lubricant). Check the flame trap and throttle body housing. I speak from experience!


Best Regards

Correction, that was Advance Discount Auto Parts. They can order them for you, all of which you shall recieve the very next day, stock up.

Start with the basics, air filter,new plugs, distrib cap and rotor,all Bosh parts. Then go to the throttle body, take it off and clean the heck out of it.
Bet the 02 sensors are bad also, don't know if this model has a knock sensor or not, but they can raise havoc as well, there are your starting points.
Good Luck
Poolman