I'm running out of ideas here, initially I thought I had ignition issues, which I did on some level, (car as 235k miles , it has minor issues with every system after plugs/wires/coil/rotor/cap things didn't improve so I moved on to fuel...

Problem is that I had major surging idle, even stalling at times, NO power, couldn't rev past 2k , some backfires etc...

I then found a suggestion to try and pull fuse #11 (in-tank assist pump), which I did and to my amazement the problem went completely away.

I then replaced this in-tank pump, reinstalled fuse, made sure not to reverse the +/- on the pump and checked everything else like cracked hoses while I was in there , reassembled it all and I still have the same exact issue

the surging idle is back, the power loss is very substantial, can barely drive the car uphills and like I said the second I turn the car off, unplug that fuse and start back up it's all good... wtf?

1) did I get another bad pump, if so, how do I test it??? the box it came in was quite beat up so I wouldn't rule that out. If I disconnect the fuel rail from fuel line and pull fuse 1 for main pump, will I see a fuel trickle from tank pump??

2) Any other ideas? could fuel relays cause this too?

3) since the wiring for tank pump is easily accessible, which wire should I check for power to make sure the computer/relays is trying to power the in-tank pump?

thanks in advance for any help you can give me. Car is 94 volvo 940 2.3 non-turbo

 

This makes no sense. If you ELIMINATE the in tank pump by removing the fuse and the problem goes away, then why don't just run the car like that? Right! The car is designed to run with in tank pump, so there is something not right with your testing procedure; and the symptoms you have don't point to a fuel pump problem.

Fuel relay won't cause this.

 

Pretty strange, I tried the same thing by pulling the in tank, #11fuse and it ran for about 15 secs before stalling.
Not sure why stopping that pump would help?

 

you are incorrect in the assumption that car won't run normal without the 2nd pump, it's just an assist pump, from what I read the car will be fine without it if you don't let the tank get too low, in the summer you may get some no-start conditions when it's hot but overall it will run just fine but it puts higher stress on main pump aka it will burn out sooner, sometimes highspeed/hirevs may also be affected but I don't tend to do standing mile in a 940 volvo so...

 

it shouldn't have stalled, maybe you were low on gas and main pump had a hard time, but it should have worked ok.

 

this are quotes from volvoclub.org.uk

Fuel Pumps:

Most 700/900 series four cylinder cars with Bosch injection systems have two fuel pumps: an intank pump which is low pressure and merely lifts the fuel from the tank and sends it to the main pump under the drivers seat. The latter is a high pressure pump capable of going to 80 psi or so, modulated by the fuel pressure regulator which keeps pressure around 35 psi by allowing excess fuel to circulate back to the tank.

also

Fuel Pre-Pump Problems & Diagnoses. Note: 94/95 940 and all 960 cars lack a pre-pump; they have one main in-tank pump instead. [From RPR ; illustration copyright and used by permission:] 7xx/9xx Volvos (except later 940s) are fitted with a fuel pump inside the fuel tank. This pre-pump, also called a primary or feed pump, moves the fuel out of the fuel tank to the main fuel pump under the driver's seat. This prevents fuel starvation problems that may occur when the tank is less than half full. If the main pump has failed, the car will not run. If the pre-pump is inoperative or if the junction hose has failed, you'll have fuel starvation240 Version of Fuel Pickup and In-Tank Pump symptoms. These may include hard starting, inexplicable poor idle, louder than normal main fuel pump noise and a loss of power under load. To check the operation of the pre-pump without removing it from the tank, locate the pre-pump fuse. Remove this fuse with the car idling. You should be able to detect increased noise from the main pump and decreased vibration of the fuel tank itself.

 

I will happily do a video and put it on youtube tomorrow I'm baffled by it as well, all I can think of is that it has to be somehow fuel pump related :shrug:

 

I think the reason why it may help with idle/running is that if the in-tank pump is failing or malfunctioning it may be restricting flow of fuel so the engine starves, I am guessing that without power applied to the in-tank pump it stays "open" and main pump does all the work of pulling gas out of the tank through the disabled in-tank pump.

question is... mine is new, am I just unlucky or is it something else?

 

I can only tell you the result. 3/4 full. Tells me my tank pump is working properly though.
I wouldn't understand why Volvo would put a dual fuel pump system in these cars unless they felt the need for it.
Trying to suck fuel through an electric pump without power just doesn't work effectively, as noted in your bulletin. In my case, it just stalls and my forward fuel pump is relatively new.

 

that's weird, literally everything I read says the cars should work just fine for a while under right conditions, well pre 95 that is, after they had 1 pump only.

why they did it IDK, I'm guessing it was cheaper overall and probably better experience for the driver, two quieter pumps > one louder ones maybe?

 

I know these cars are supposed to run without the in tank pump, like the write up says, BUT the fact that yours runs OK with the pump disconnected completely is what is weird. I still think that your problem is not either fuel pump but something ignition related. You should test the fuel pressure at this point to make sure...

 

I am not sure I am qualified to answer because I can't find the problem with my sons care for the life of me but while I was researching on here about pumps I ran across a write up where the author said his aftermarket pump was wired backward. You may try pulling the line to it, run another line to a can and turn the key to position 1 which should start that pump. I replaced my pump today and I still can't hear it run. Checked voltage and in key position 1 it is 10 volts, when I start the car it drops to 0V Not sure is this is how they are supposed to work, just prime the main pump or if I have other issues. I checked with two different pump relays, same result.

 

If you have installed an aftermarket fuel sending unit, then chances are the wires could be backwards. When i went to install one, I checked the wiring first. I found that the not only the polarity of the pump wires were switched, they had also switched the pump power wire with the fuel level sending unit. Fortunately I was forewarned and checked the wiring prior to installation.

 

hmm so I may have to lie in the trunk for 2 more hours? damn sedans, never have I wished I had a wagon so much...

ok that makes sense, it may be that new pump has reversed "posts" which would royally suck, I will def see if it's pumping first I guess.

I'm going out to shovel out the pile of crap and I'll make a video of my mess, why not , at least someone can laugh at my misery

 

Turning the key to the number 1 or 2 position will not do anything to activate the fuel pump. The pump will only activate in the "start" key position and only for a second. Once the Hall Sensor or CPS senses the engine running, the fuel relay will close and the fuel pump will operate with the key in the run or #2 position.
If the fuel pump operated in the key 2 position and the engine wasn't running, you'd burn the pump out pretty quickly.

 

so I head out there to make the video and wouldn't you know it, I think you guys are right about it not being fuel pump related now well for one replacing the fuel pump didn't help which is sort of a hint... lol , but it's doing weird stuff now, for example at one point while sitting in the driveway and idling WITHOUT the #11 fuse I was able to make it surge just by pressing the brake pedal, with pedal down it would surge, with pedal up it would stop. It has always acted like a vacuum leak, I just could never find one so quest continues I guess.

I have a vacuum gauge, will check that next when snow apocalypse ends, I will buy the adapter so I can also check fuel pressure and go from there.

may be the brake booster failing I suppose, I think I can take the vacuum hose off and cap it on both ends and that would eliminate that right? (wouldn't have assist brakes at that point).

Any known issues with removing ALL of the fuel vapor/charcoal canister junk too? I see it has few vacuum/vapor lines that go into the throttle body/intake. I will definitely cap all of that off as well and probably rip it right out for now.

There's always the possibility of something dumb like intake gasket, but I wouldn't think that would come and go...

Now WITH the #11 fuse in and both pumps going the problem is much much worse, surging is very bad, car is undriveable, can't rev, no power... so whatever that means IDK... I can't quite think of how extra fuel pump would make vacuum leak worse, if I Recall it had a big hose that was the fill-up neck, then obviously fuel outlet, fuel return hose but I think there was one more.. I'm guessing for the vapor system, but if the vapor system is hooked to vacuum and that line has a hole/crack somewhere maybe the pump being activated makes it worse??? total guesswork on my part, I guess vacuum gauge will tell me more once I Get to it.

 

An easy way to check for vacuum leak, although not the safest but one I have been doing for years is to spray starter fluid around the vacuum hoses while the engine is running but still cold. If the engine revs higher in a certain area you have found your leak.


Not trying to hijack your post but responding to R2Millers, are you sure this is the case with the in tank pump? I believe you are right with the primary pump mounted under the chassis but the in tank pump I have read should come on in position 1. Problem is mine shuts off in position 2

 

the behavior of pumps has changed throughout the years, some would go on in pos 1 , others not, it depends on your car/ignition system/what year...

 

position I is accessory, should be NO fuel pump action at all.

position II is run. later cars will run the pumps for 1-2 seconds when you switch the key to II, but then stop until the engine actually turns over.

as long as the engine is moving, the pumps should be running continuously, but then when the engine stops, the pumps shut off 1-2 seconds later.

 

Thanks Pierce.

Edited my post above.