S70 2.5 20v power loss / fuel system layout

[foggyjames]

Hi all,

I'm getting intermittent severe power loss and (very) occasional poor starting. The power loss is as if I've closed the throttle to say 25% once I get to 3000-3500 RPM, and it's very slow to rev past about 4500rpm..picking up a little towards redline. This is most pronounced in first gear, but also affects second. Power seems to be down in general...but it can be hard to tell. In my experience, if it feels like it's down...it is.

This all started when I put a replacement engine in (after a cambelt failure). The new engine was slow to start on the first crank, and suffered a stuttering misfire at low RPM / high load which was cured by fitting the (pretty new) ignition components from the original engine. The power loss was there from day one on the new engine, and never there on the original. It's getting worse as time goes by. Once in a blue moon the original setup would give poor starting...perhaps once or twice a year? The only sensor not swapped over is the throttle position sensor. The power loss does seem to 'hiccup' with throttle position, so I am very tempted to try swapping the throttle bodies (not the TPS...that sounds far too frought with adjustment difficulty!).

The car is a 1997 S70 2.5 20v on Motronic 4.4. It has the single fuel line layout...no return line, and no (conventional) pressure regulator...only an unidentified item which looks more like a pulse damper to me.

Confession time: The fuel filter is 40k overdue. I forgot all about it at the 100k mark. A new one arrived today, so as soon as I get an hour or so free when it's a) dry and b) light outside...! However, my experience has been that a filter is unlikely to cause this...although of course I will be doing it ASAP...

The power loss is quite 'smooth'...it's not like a misfire. I suspect fuel delivery is the problem. I've already tried a known good AMM, and the problem persists.

If I was coming to this fresh, my finger would be firmly pointed at the fuel pressure regulator...but...how the hell does the system work on these cars? There's no return line...and no "conventional" regulator. I can't think of another car like it off the top of my head. VADIS is very vague on the subject, listing no part numbers for my setup...only for a ME7 car with a single line. Assuming the filter doesn't cure it, my next port of call is a fuel pressure test.

So...my questions...
1) How the hell does the system work? Is that thing on the rail a regulator, or a damper? If there's no regulator, how is constant pressure maintained?

2) Does anyone have any other thoughts on this problem...something I might be missing?

cheers

James

[Chris_Rogers]

Going from memory I think the fuel pressure regulator is at the rear of the car near the fuel filter.

As this is a 20V NA car have you had a look at the SAS valve and the operation of the variable inlet mechanism?

http://volvospeed.com/Repair/airpump1.php

[foggyjames]

Thanks for the reply, Chris. I'll take a good look under there when I'm changing the filter and see what I can find! A fuel pressure test should highlight any problems in that area, I think.

My car doesn't have an SAS valve, or the variable inlet setup. That only came on the earlier 850s with the "big port" head, as far as I'm aware. I'm afraid it's much more boring than that!

cheers

James

[Chris_Rogers]

Found this on fuel distribution:

The fuel tank holds 73 liters. The filler pipe is routed outside the passenger and cargo compartments, and has a check valve to prevent spillage when refuelling.
The fuel filter is located beside the right hand rear axle mounting bracket, beside the roll-over valve. Downstream of the filter a three way unit with a pressure regulator maintains pressure at 300 kPa, by returning excess fuel to the tank. Only one fuel line runs from this three way unit to the non-return fuel rail.
A vent valve upstream of the filter and a suction connection on the fuel rail are provided to prevent spillage, when replacing the filter for example.
The fuel filter and pump have quick-release connections. Disconnect by pulling the outer connector sleeve to disengage a snap ring on the inside of the connector.
The fuel pump and level sensor are separate units. The level sensor is tubular and its resistance increases by 5 Ω per litre.

[foggyjames]

Excellent - many thanks Chris. So it looks like there *is* a regulator...it's just in a funny place! The fuel rail is kinda like an "outpost" (as some carb setups are plumbed)...most bizarre!

I think a fuel pressure test is in order...

cheers

James

[foggyjames]

Bump. Looks like I might have found the source of the problem.

I changed the filter earlier. The regulator certainly is at the rear...right in front of the filter! How wierd...! However...no change, sadly.

I got back from an evening at my mate's place just before midnight, a bit grumpy at facing closing the day off without having cracked it...so I decided to give a throttle body swap a go. The TPS was the only sensor still being used which came from the replacement engine (i.e. was not "known good")...and adjusting the throttle input seemed to affect to what extent it was losing power. I chose to swap the entire throttle body so as not to have to worry about aligning the TPS.

Anyway...long story short...following the TB swap it was pulling like a train in first every time on the test-drive, and felt to have a lot more go in every gear. It's early days...but I'm feeling good about this! I shudder to think how lean it must have been running...

cheers

James

[misa]

You have fuel pressure damper,not regulator,and it's located at fuel rail.Here's the picture:

[foggyjames]

Thanks for that...great document! There's a regulator next to the fuel filter, as well as the damper. What a wierd place to put it! I took a picture of it earlier, but don't have it off my camera yet.

cheers

James