View Full Version : Bypassing the mechanical fuel pump

05-21-2012, 08:15 AM
Hi guys - so while dong a test drive yesterday to see if I had sorted out the OD issues the TR6 started to stumble and loose power. Quick pull of the choke, stumbling went away, so it was pretty easy to confirm it was fuel. Pull to the side of the road and find that the fuel filter is dry, despite the fuel tank being half full. So that suggests the mechanical fuel pump being the culprit.

Run home (need the exercise anyway), quick trip to drop my daughter off at Brownies, and then I installed an low pressure electric fuel pump in the engine bay. Let it prime for 15 seconds and then she fired right up and ran great.

Due to being on the side of the road and not having much spare time I connected the fuel pump so it pulls through the fuel filter and <span style="text-decoration: underline">pushes</span> through the presumed dead mechanical pump. I'll put in a proper mechanical pump but that has me wondering if there would be any issues in running it like this for a while?

What do you think guys?

05-21-2012, 08:44 AM
My experience: when the fuel pump died (in heavy traffic on Memorial Day weekend, in the hottest week in ten years ...), I had a friend pick me up and drive me home to fetch an electric pump I had never used.

Drove back to the "site of the embarrassment", and spliced it in line, in the engine bay. Didn't feel like crawling under the car in the Memorial Day conditions.

The pump pushed fuel through the old pump and into the carbs. Engine ran great and got me home without problem.

But ... after a week of sitting in the garage, the fuel had drained back down through the system, and into the tank. Thus the line was empty.

The electric wasn't strong enough to pull fuel through the "empty" part of the line, to be able to push it onward to the engine.

The pump had been designed as a "push" type, to be mounted just forward of the tank. In my emergency setup, when the line was already full of fuel, if worked fine. But not when the line was empty.

My two cents.

05-21-2012, 09:06 AM
No, it will not hurt anything to run both pumps in line. We used to do this as a rule in the old Chevy's I used to race. The mechanical pump just has a couple check valves that the electric pump will push the fuel right on through.

The only thing to watch is the filter in the mechanical. If it was the original problem (as in clogged) is is still going to be a problem now.


05-21-2012, 09:36 AM
The only thing to watch is the filter in the mechanical. If it was the original problem (as in clogged) is is still going to be a problem now.

Ditto the diaphragm.

05-21-2012, 09:47 AM
Thanks guys.

This is all a bit theoretical as I'll replace the mechanical pump, but is there a risk of blowing out the seal and getting fuel into the block? Or, since the pressures are the same, is that a non-issue?

05-21-2012, 09:53 AM
"blowing out", no. But if it fails from old age or perhaps not being compatible with gasohol, it can still leak fuel.

Geo Hahn
05-21-2012, 11:24 AM
I carry an electric pump as a 'get-home' spare but I always bypass the mechanical pump when I use it. I connect it to the (undone) soft line that goes into the pump fitting and to the hard line that goes to the bowls (have long 5/16" to 1/4" hose on the output end). Of couse that set-up which is easy on a TR3 or 4 may be more cumbersome on a TR6.

My reasoning is that a failed mechanical (depending on the failure) might allow fuel to get pumped into the crankcase. Perhaps that is not possible?


05-21-2012, 12:12 PM
It normally spews out the vent hole on the pump. But, with enough pressure and a large enough hole in the diaphragm, it is possible.


05-21-2012, 01:29 PM
I had the opposite problem (although on a TR3A rather than TR6). The diaphgram was only leaking a small amount of fuel, but it wound up in the crankcase. I only knew there was a problem when the oil level went up instead of down.