'98 S70 Jerky hesitation Topic is solved

Post by **dosbricks** » 12 Apr 2017, 21:31

I believe the on-demand system varies the fuel volume by adjusting the speed the pump runs by varying the duration of a pulse in the electricity to the pump. That can cause a pulse in the fuel pressure which is not good for the function of the injectors. The pulse damper is just like a cushion to absorb that shock being transferred to the fuel entering the injectors.

Post by **cn90** » 12 Apr 2017, 21:43

Sorry I didn't read the entire thread but any CEL codes?

My past experience with "jerky hesitation" was from a bad MAF.

Post by **misha** » 13 Apr 2017, 03:30

dosbricks wrote: 12 Apr 2017, 21:31 I believe the on-demand system varies the fuel volume by adjusting the speed the pump runs by varying the duration of a pulse in the electricity to the pump. That can cause a pulse in the fuel pressure which is not good for the function of the injectors. The pulse damper is just like a cushion to absorb that shock being transferred to the fuel entering the injectors.

Nope...pump have constant speed and fuel pressure regulator is there to maintain adequate fuel pressure when it's needed.
Pulsing damper is there to apsorb pulses which are created by injectors.
All Motronic 4.4 management have fuel pressure regulators and return line in fuel filter area behind passenger's side rear wheel.

Post by **misha** » 13 Apr 2017, 03:34

theWIFES_S70 wrote: 12 Apr 2017, 19:01...Misha, I've always been curious, what exactly does the fuel damper do? Is it something we should all start replacing as we approach year 20 on our cars?

It absorbs the pulses which injectors are making.
When fuel pressure damper fails,injectors becomes very loud.That's the only side effect besides obvious leaking.
No need to replace it if injectors don't make loud ticking and if it's not leaking.

Post by **dosbricks** » 13 Apr 2017, 14:11

Cn90, no codes. It has a Bosch OE MAF installed about 2 years ago.

Sorry misha, I thought I might be going out on a limb there. That info was from reading a generic description of how on-demand systems work--not Volvo specific.

I'm leaning toward the idea that my problem is caused by a weak and failing fuel pump not keeping up with demand. I have the new Rockauto pump coming and a pump seal ring, so next step is to it and see what happens.

It also occurred to me why the fuel pressure relay from the JY did not work (face palm

). That car had been smacked on the left front, rolled over, and the air bags were deployed. So the relay is in the system to cut off the fuel in the event of an accident, probably by cooking itself. $2.50 down the drain.

Post by **bmdubya1198** » 13 Apr 2017, 14:53

I beg to differ on that M4.4 comment about the fuel pressure regulator in the fuel filter area... my car is M4.4 and I have the regulator on the firewall. It seems not many cars have that setup, which explains why it was so hard to find a new regulator when I THOUGHT mine had gone out.

Anyway, this thread brings up a question about my fuel pump... if I get little hesitations, usually on braking, when I have 1/4 tank or less, could my pump be on its way out? My car's little hiccups/misses/hesitations have gotten better and less frequent since I replaced my crank sensor, but I have noticed when they do happen, I'm usually lower on gas (but not always). It's similar to what dos was describing, except it feels like a brief misfire or loss of power. I'll be sitting at a stop light and the car jerks a little and the tach quickly goes down and back up about 200-300 RPM.
I've already replaced the fuel pump relay (like-new KAE from Pull a Part... it was there, had to get it), fuel pressure regulator, crank sensor (it had a BAD loss of power at one point throwing multiple misfire codes and a crank sensor code), and it has all new ignition parts (plugs, wires, cap & rotor). Although it's worth mentioning the wires are NGK, not Bougicord.

Post by **bmdubya1198** » 13 Apr 2017, 14:54

dosbricks wrote: 13 Apr 2017, 14:11$2.50 down the drain.

It's Pull-a-Part, go get your store credit!

Post by **misha** » 13 Apr 2017, 15:12

bmdubya1198 wrote: 13 Apr 2017, 14:53 I beg to differ on that M4.4 comment about the fuel pressure regulator in the fuel filter area... my car is M4.4 and I have the regulator on the firewall...

On a turbo Motronic 4.4,YES.
I was talking about non-turbo Motronic 4.4.

Post by **bmdubya1198** » 13 Apr 2017, 16:03

misha wrote: 13 Apr 2017, 15:12
bmdubya1198 wrote: 13 Apr 2017, 14:53 I beg to differ on that M4.4 comment about the fuel pressure regulator in the fuel filter area... my car is M4.4 and I have the regulator on the firewall...
On a turbo Motronic 4.4,YES.
I was talking about non-turbo Motronic 4.4.

Ah, that would make sense.

Post by **cn90** » 13 Apr 2017, 16:25

Re Fuel Pump RELAY: I thought the design is when a crash happens, the computer senses crash and shut off the supply to terminal #15. I maybe wrong though.
Anyway, the best way to rule out a bad relay is jumping #15 and #87 with a jumper wire (with the spade connectors at each end).