1995 Volvo 960, won't accelorate

Fairmount · Post by **Fairmount** » 18 Jun 2011, 08:51

My 1995 Volvo 960 will not accelerrate. It starts okay. I replaced the plugs and accelerator control module and that did not help. I recently took the car to a local repair shop. The mechanic said the car needed to have the mass flow sensor replaced, fuel system cleaned, and the computer reprogrammed. He put an OEM Volvo MFS on it, cleaned the fuel system, and reprogrammed the computer. Ultimately, the car was no better. When I picked it up he said he was definitely sure the car needs a new MFS, but he said that the car needs more than that to fix it. The mechanic checked the fuel line pressure and he said it appeared the fuel pump was funtioning fine; also, he checked numerous other things. So, with all that he said he could not figure it out. However, he was honarable gentleman and did not charge me a single dine despite spending several hours on it; he said the only thing he wanted was to know what the problem was when I find out. (this garage will get good word of mouth recommendations from me.)

The mechanic mention from time to time, the car will accelerrate just fine, but quickly go back to accelerrating poorly in the same accelerration cycle. I have experienced the same condition over the past month. During accelerration it feels light and quick for a few seconds and suddenly in the accelerration cycle it will revert back to running poorly.

So, I am at square one hoping to get some leads as to what to do next. Could it be a vacuum leak; could it still be the fuel pump; orm fuel regulator? Any help would be greatly appreciated.
Stan S.

Post by **jimmy57** » 18 Jun 2011, 12:34

If I were working on the car the first thing I would do given what has already been tried is to put a fuel pressure gauge on it and drive it to see what fuel pressure does when driving. It should be around 34-38 idling and go up with increased throttle opening. If it falls when you drive it and load engine then there is fuel delivery problem.
If this checks out then I would verify oxygen sensor operation.
The thermostatic door in air cleaner housing can stick in hot position and make the car weaker in hot weather but I would not describe that as "won't accelerate".

DanR · Post by **DanR** » 18 Jun 2011, 15:38

A quick way to test a mas, is to disconnect it, the computer will go into limp home mode and it usually runs better if the sensor was bad with it disconnected.

I agree with 57 on the fuel pressure reading. Another item that can be the cause is the coolant temp sensor. It could be signaling to the computer that it is colder than it really is.

DanR '94 964 356,000 miles (122,000 on the new engine)

lindy8_man · Post by **lindy8_man** » 20 Jun 2011, 16:27

Well, these guys probably know a lot more about this car than I do. I had a vacuum leak under the dash a year ago 98 s90 210K... caused all kinds of funny things including bad acceleration and no cruise control. If your ventilation system is vac controlled you coul have as many as 3 bellows under the dash that coul leak. Yes sometimes the leak could slow down, and return. I could hear these leaking within "the suitcase" (the ventillation box behind the dash).

do you have cruise? there is a big bellows under the hood that controlls the cruise. My volvo guy says these go bad frequently... the bellows will likely fail before the hoses will.

Fairmount · Post by **Fairmount** » 21 Jun 2011, 05:18

Thanks to all for your advise. With regard to the cruise control, it is functioning. I have not done anything yet in the way of repair (I have a second car to drive in the meantime), but I am planning to get started on it this coming weekend. I will update on my progress.

Could this problem be related to a "knock sensor". A friend told me that if there a knock, the computer can retard the timing.