Could my leaking fuel pump be causing awful mpg?

WarrenL

Active Member
This has been touched on elsewhere, but I'd like to know if others agree with the theory, first put forward by Chris York, that my leaking fuel pump is causing awful fuel consumption figures. I just topped up Brown Rover after 100 miles of mixed urban/local motorway driving, and it worked out at a thoroughly awful 16 mpg. The last open road trip, also 100 miles, was a very disappointing 20.5 mpg. Had the car still had the BW35 I'd have expected 25 on that trip, but I had the bonus of an overdriven top gear by then, if not yet the torque converter lockup.

The block is rather damp looking behind the fuel pump, and there has been some dribbling onto the cross member, but it's hard to tell just how much is escaping. Could it really be costing me a good 5mpg?

To a certain degree the question is moot, because I have a new pump on its way, but I'm worried in case it doesn't entirely solve the problem. The car was tuned to within an inch of its life just before the BW blew, and it seems to be running really nicely - I'd have expected somewhat better.
 
Hi Warren . Have you pulled the dip stick and had a sniff of the oil ? If you can smell fuel it'll be pumping it from a split diaphragm into the sump , Not good for your motor let alone the consumption . If it's the case i'd leave it parked up until the new pump is fitted and the oil changed .
Sorry if i've just written a load of stuff you already knew :D
 
Hi Warren,

You'll only know for sure once you have eliminated the pump as a possible cause. A leak will always have a negative impact by its very nature. Just one other area that you can check in the meantime, the vacuum advance module. If it is not working then fuel consumption can increase by around 10%. With a piece of hose attached to the module pipe, if you can blow in without restriction or cannot hold suction against your tongue, then the module requires replacement.

Ron.
 
Oil's fine, Stina. I just checked it yesterday for that very reason. The pump had a new gasket fitted in March and the diaphragm was in perfect condition, and it was the stripped inlet thread that led to the present situation where the fuel is leaking back out of the olive. I just didn't think it could be enough to blow my fuel consumption to the tune of 5 or more mpg, but Chris York, the demon Richard and a couple of non-Forum sources are all of the consensus that it is the probable cause. However, I will check the vacuum advance module on your advice, Ron.

Right when I want to see what sort of effect the new transmission has on consumption! Just my kind of luck...
 
Of course, but knowing intimately the car's recent history, the leaking fuel pump is the only immediately obvious possibility. That's why I wanted to know if others have experienced the same symptom. Fuel pump has to be repaired/replaced in any case, so if the problem isn't resolved afterwards I'll know to look further. And I'll check the vacuum advance in the meantime.
 
It's Christmas morning. The kids have toys and DVDs and are ignoring me. 'Er indoors has a new smartphone and is ignoring me. Even the cat is ignoring me. Only Brown Rover is asking for attention, so I went and crawled underneath to check a few things. I made sure the fuel tank olives were tight and not leaking; I popped a couple of hose clamps around the fuel lines (a tiny bit of ooze where the return line meets the steel pipe coming out of the sender unit - better to be safe than sorry) and finally, Ron, I checked the vacuum advance. If I pull the pipe off the carburettor and suck on it, the dizzy plate moves back and forth, but it does require a decent bit of suck on my part. It rocks back and forth about 2-3mm (maybe a bit more) and if I move it with my finger I get just a little bit more than that. Can't blow into the pipe at all. Solid resistance. Sound about right?

UPDATE: I just prodded the lifting pins on my warmed up carburettors and I think that the LHS is tweaked a little rich, and the RHS is a little lean. Can I adjust these without totally f***ing things up?
 
Yes.... You can tweak them. Just count the number of turns of each screw and direction and you can always put it back.
 
Dunno how much it might have to do with my poor mpg but I might as well sort it while I'm on the job.
 
Hmmm. Tuned within an inch of its life you said... Now I wonder who did that? If it wasn't you, then chances are you can do a better job....

Chris
 
Yeah, within an inch. But an inch still allows some wiggle room. I'll leave the person unnamed, but it wasn't me. And I could have sworn we checked the lift pins at the time... anyway, it was no longer belching black clouds of unburned fuel out the back, it idled, it revved smoothly, and it finally had some power back. Furthermore, the fuel economy was subsequently considerably better than it has been just lately. Go figure. I'm just wary of fiddling with the carbs because I don't really know what I'm doing. If I twiddle the mixture screws until the lift pins are producing the right result*, will I be screwing up anything else? And could this be contributing significantly to my depressing fuel consumption?

*Correct me if I'm wrong, but if I lift the pin and hold it, I should have a little rise in engine speed and then a drop to the original speed. Is that right? Do I try it with or without the air filters on?
 
I'm personally not a fan of the lift pin method.It's probably OK if you're doing it all the time and have got your eye in, so to speak, but done once every blue moon I don't think it really helps. A colour tune plug swapped around the engine - or two, one in each of the carbs cylinder groups is much more user friendly. And no - with the air filters in. Set mixture and idle with the link between the carbs disconnected. Once you're happy then re-connect it, adjusting it for the new settings. Then make sure you get full throttle on both carbs. If not by any sort of margin, then go back and try again!

Chris
 
Sure, if you have a Colortune kit, but the lift pins are all I have to go on! So, checking full throttle is engine off, and see that the linkage moves opens both carbs the same? How about setting the idle with two carbs?
 
i do it by trial and error. The Colortune are only a guide as their 'correct' colour runs weak in real life for me... I have tried to use it to get the mix even between both carbs but ended up listening to the lift pins and throttle response as it works better for me. For idle i tend to listen to me k&n style filters up close. Because they're noisier differences in air flow are pretty obvious.

Rich
 
I'm sure John used a Colortune or similar to set things earlier in the year. Car seems, to all intents and purposes, to have been running beautifully, but maybe I'll start with a gentle tweak of the mixture screws according to the lift pins.

Now, demon Richard, I take it there's a bush technique to setting the idle by listening to the carburettors, but it's the idle screw on each unit that is confusing me. Do I set one idle screw and then adjust the other until it sounds the same, or something?

As an aside, the carbs were pulled apart, inspected, cleaned and equipped with new seals, gaskets, etc earlier in the year, just before that last tune. Is it possible that the few hundred miles driven since then is enough to bed things in and upset the fine tuning?
 
I set one idle screw and then adjust the other until it sounds the same, or something?

exactly that. Takes a few goes but eventually you get it spot on. I have hs6s ...they drift easily... HIFs not so easily i believe...
 
Not really happy. I had a bit of a fiddle, but I am not at all comfortable with doing it. I noticed that the port carburettor is only a 4 half-turns from fully clockwise (fully rich, is that right?) but even if you take it all the way it still stutters when the pin is lifted (too lean?). And it's hard to know what to do with the two idle screws. I'm sure it's easy if you know how, but it's difficult when you're stabbing in the dark. I am going to give the local club's P6 man a call - apparently he's pretty good at setting up the carbs.
 
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