I got the car back from the repair shop, 1987 Volvo 780, and it runs rough as shit on start up in the morning until it warms up. I think it may be the cold start valve but im not sure????? Ever since they replaced the fuel pump and filter my gas mileage has gone to shit. I got 80 miles to a half a tank. What could be causing the bad gas mileage??

 

is your throttle body clean...where i work has this 3 part fuel system cleaning thing where we scrub the throttle body and run a machine which injects a cleaner into the throttle body and has a fluid for the gas tank which cleans out your injectors and everything. let me know if you want any details on it or come by and get it done, from what i've heard it really improves the gas mileage and how your car runs

 

I had thought about taking the throttle body off this weekend and cleaning it. I would like the details like how much does it cost to have what your talking about done?

 

Also try replacing pcv. Once they go bad it can hurt gas milage also.

 

i believe about 60 bucks...i could get you like a 10% discount most likely. i'll check when i go to work next

 

Get your parts from Autozone, they give all military a 10% discount.

 

a PCV valve won't affect mileage that much, they're a small improvement and are mostly for emissions. This sounds like something like a O2 sensor, MAP/MAF sensor, or TPS sensor, it's definately something that is throwing the ECU fa curveball.. I don't know crap about Volvos though, so I don't have any insight on which one it could be. Does it smooth out after it warms up, or does it still run rough, just not as bad?

I'd be interested to know if your new fuel pump is flowing a lot more than your old one and the pressure isn't getting regulated properly. If it's only been doing this since the filter and pump replacement, I'd look there first. If there is a convenient place on your fuel rail to tap a fuel pressure gauge that may tell you a lot.

 

The car does smooth out after warm up.

 

The ground wire to my TPS was fucked up and totally screwed my gas mileage. Actually it felt like I was driving a four-wheeled seizure.

 

Interesting. Does your temp gauge read correctly? Fuel system aside, there are a few sensors that can cause stuff like this, but again, I'm not sure how much they would affect the gas mileage compared to what you're seeing.
If the coolant temp sensor was bad, it would run like shit, but you'd see your temp gauge act wacky. I just can't imagine what would make your mileage go down so bad, most sensor problems will make it run like crap but not affect mileage that much. Most sensors that are bad will cause it to run poorly all the time, not just under temp conditions (except for the coolant temp). Does it run like crap only at idle or under load also?