Weird effects of fuel level on starting/running

[AllHailKingVolvo]

I’ve recently found my 744 (B200F) has been reacting oddly to changes in fuel level.

It’s currently in for a new in-tank fuel pump anyway, but unlike the usual weak ITP symptoms of poor running below half a tank of fuel, I have the opposite problem!

When freshly brimmed with fuel, the car is hard to start (often taking 30 secs or more of cranking before stuttering into life) and runs lumpily, hesitating under power (as it did previously before I gutted the cat).

At 3/4 full, the problem is gone and the car starts normally and runs perfectly, as it does down to close to empty (the fuel gauge is temperamental so I don’t take chances!).

What could be going on here? Some kind of vapour lock or blocked tank breather?

[Laird Scooby]

'Tis a weird one Phil! A blocked breather would normally cause cutting out after driving for a while and a vacuum was created in the tank. Fresh fuel, especially a full tank, will give extra pressure with blocked breathers in the fuel supply.

Only thing that comes to mind (and i'm not convinced) is maybe there is some condensation in the tank and the extra weight of a full tank of fuel is forcing more of it to the bottom of the tank and holding it there. However, condensation usually presents itself as a problem when there is less fuel in the tank and would be present almost all the time.

Does yours have any charcoal canisters for vapour control? I don't think i've seen any reference to them in the HBoF or anywhere else but its' possible.

Might be an idea to see what it's like after having the new in-tank pump and see what happens.

[AllHailKingVolvo]

It’s a weird one isn’t it? The condensation could be a factor, especially as the car sat for some time before I owned it. I’ve put 6000 miles on it since then though-maybe it’s worth giving it a dose of meths to see if that clears it. It’s been doing it consistently for a couple of weeks now, I think it was less noticeable beforehand with the blocked cat holding it back but is more evident now it has some proper grunt!

I have a charcoal canister up front near the NS headlight, but nowhere else as far as I can see.

[Laird Scooby]

Originally Posted by AllHailKingVolvo

It’s a weird one isn’t it? The condensation could be a factor, especially as the car sat for some time before I owned it. I’ve put 6000 miles on it since then though-maybe it’s worth giving it a dose of meths to see if that clears it. It’s been doing it consistently for a couple of weeks now, I think it was less noticeable beforehand with the blocked cat holding it back but is more evident now it has some proper grunt!

I have a charcoal canister up front near the NS headlight, but nowhere else as far as I can see.

If it was happening all the time i'd say condensation, plugs, HT leads and so on. If yours has the purge canister/charcoal canister i think i'd investigate the purge valve and wiring to it.

I'm not sure if it works the same on the Volvo but on my Rovers, what happens is the canister catches the fuel vapours from the tank that would otherwise escape to atmosphere and stores them in the charcoal. When driving with the throttle open, it opens the purge valve to let air into the canister and another to connect the canister to the inlet manifold/air intake. This burns off the vapour, saving fuel and preventing raw petrol vapour reaching the atmosphere.

That's the theory anyway!

Sorry to say i don't really know anything about the system on the Volvo, the only Volvo i ever had with a cat and Lambda sensor and charcoal canisters was an ill-fated L reg 440Li which was basically a Renault Migraine in a pretty (? ) dress - less said about that the better!

My guess, based on what little i know of the charcoal canister/purge system is the extra vapour from a full tank is reaching the charcoal canister and getting past the purge valve to the manifold/air intake.

Perhaps a starting point for further investigation, maybe a red herring but worth looking at as you may discover something else causing the problem.

[AllHailKingVolvo]

I’ll check out the charcoal canister-sounds like a likely culprit!

The garage sorted out my rusted and siezed rear shock absorber bottom bolts and fitted the shocks i bought a while back which has made the car ride SO much better and has straightened out the slightly saggy back end too, which is great news.

Unfortunately they couldn’t do the pump as the top of the sender unit “lid” as it were has corroded to the point where they weren’t confident that it would come apart without disintegrating...they’ve asked me to source a secondhand sender unit so they can do the job without the worry of the car becoming a non-runner in their yard with a broken sender unit, which is understandable. It’s pretty crusty so a I can see their point. Luckily I have a local Volvo breaker in Piddletrenthide so hopefully they can supply!

[Laird Scooby]

Shouldn't be too difficult to find a secondhand one of those Phil and while i think of it, invest the £11 or thereabouts on a new seal for it :



I'll find you the part number tomorrow, they're pretty much universal across the 7/9xx range and have a tendency to leak once they've been disturbed.

Just to make sure i've got it clear in my own mind, they do actually mean the sender unit and not the access plate?

[Laird Scooby]

Sorry for the delay Phil, part number for the seal should be 1367623, get your local Volvo dealer to double-check but i'm sure that was the number i used.

Any progress on the rough running on a full tank?

[AllHailKingVolvo]

Originally Posted by Laird Scooby

Sorry for the delay Phil, part number for the seal should be 1367623, get your local Volvo dealer to double-check but i'm sure that was the number i used.

Any progress on the rough running on a full tank?

Thanks Dave! I’ll order one of those today.

I disconnected the tank breather pipe from the charcoal canister (it was disconnected when I got the car and I connected it up thinking i was fixing things!) and all seems fine now...perhaps a new canister is required.

[Laird Scooby]

Originally Posted by AllHailKingVolvo

I disconnected the tank breather pipe from the charcoal canister (it was disconnected when I got the car and I connected it up thinking i was fixing things!) and all seems fine now...perhaps a new canister is required.

Hmmmm, methinks the previous owner may have had similar troubles then!

That would point to item 23 on this diagram being the main fault then :

https://www.volvopartswebstore.com/s...rimLevel=28244



The carbon filters/charcoal canisters don't wear out as such. They absorb the fuel vapour and are meant to then purge under hardware control from the ECU at the right time. When they purge, fresh air is meant to be drawn in to replace the fuel vapour and "dry" the charcoal out. Prime suspect would have to be the solenoid valve being stuck open so it's constantly providing fuel vapour to the engine when the tank is full, possibly also when it's not full as well!

I'm sure if it is the solenoid valve, you could find another on ebay (or a breakers) that would work for considerably less than Volvo want!

[Laird Scooby]

Had a look on fleabay Phil, if it is indeed a 12v solenoid, this would probably work as a substitute :

https://www.ebay.co.uk/itm/DC-12V-Mi...e/263731714432



You'll have to check the pipe sizes etc but it would be a good place to start, about $110 cheaper than the Volvo item listed!

Food for thought anyway!