MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - What limits the power after 20 mins ?

Standard Rangerover 3.9 plant. Pulls well at 2750 rpm and above for 15 to 20 mins, increase to 3000 rpm on the straight and she stutters a little so, slow to 2500 and runs happily for 30 mins but then won't go up a shallow hill (Butser for those who know the A3) above 2000 in a lower gear. Same pattern of stuttering and perhaps a pop from the plenum. This has happened twice in a very similar pattern.
Coil, leads and dizzy amp all newish as are both fuel pumps. After the hill, she drove home normally. The LP pump is the standard modern replacement for the 1800's SUs feeding a swirlpot, the HP pump is a Bosch delivering 40 psi to the fuel rail.
I suspect fuel starvation as the likely culprit. Ideas very welcome -
Roger W

Dirty tank, dirty filters, insufficient venting ? Worth a look as easy fixes. Corroded tank is common.
Allan Reeling

Just had a similar experience with a Camry. Eventually tool it to a specialist, turned out to be the injector seals were shot, allowing air to be sucked in.

Herb
Herb Adler

Lack of fuel - pump going out or clogged filter.
Bill Spohn

Useful comment - many thanks. Stainless tank although one can very occasionally get dirty fuel here - two filters, neither very long in the tooth - Injector seals pretty old so thanks for your experience Herb. I have seen the venting issue raised here in the past Allan - I have the usual lockable cap - have never had probs before. How do you get round it ?
I still don't trust the pumps. They supposedly are good quality but even the best are made in faraway places that I haven't heard of and don't want to visit - both normally give some audible warning of failure but all possibilities need to be explored.
Thanks again.
Roger W

Roger, just get a cheep temporary plastic effort and drill a hole in it, then drive in the dry!! Have you tried some injector cleaner? Sometimes the rubber pipes on the engine side of the filters break down and restrict things.
Allan Reeling

UPDATE
Following Herb's experience with his Camry, I changed the injection seals. Very glad I did because there had been some water in the system sometime in the past and the rust needed cleaning out. So, out on the A3 for a testrun to see if the problem was solved. Ran like a dream (as rqd by law) til just North of Petersfield and accelerated out of the B3006 roundabout - left the hothatch behind and .... it happened again. Much stuttering and a slow drive off to let her cool and then a slower - actually very slow (1500 rpm) drive home. Herb - thanks but the Buick/Rover has many faults but this is one that the Toyota can claim to itself !
So - back to the process - HP pump next but I wonder if I have a lifting head gasket.
R
Roger W

Roger,
What vintage of 3.9 is it?
Take the plugs out one by one and look for an abnormally clean one.............steam cleaned!!! Early 3.9's were prone to liner lift, allowing water into the combustion chamber. Sometimes you can hear the liner tapping on the cylinder head when the engine is hot. It's caused by vertical cracks in the alloy "gripping" the liner, which in turn is caused by the metal being too thin. When the block is hot it loses it's grip, the liner shifts and water enters the bore from the surrounding water jacket. L R started grading blocks according to how much the cores had moved during casting and only boring out the ones with less core shift, therefore more metal around the liners.
Allan Reeling

Allan,
That sounds serious - this is a 93 engine - otherwise in quite good nick - I changed the plugs recently - they were a bit sooty after a period of slow running - but, I will have another look - thanks
Roger W

Roger,
With a EFi engine, slow running shouldn't produce "sooty" plugs. After all the whole advantage of injection is that the mixture delivery should be spot on in all circumstances. However I haven't a great deal of experience on EFi but it does sound like you have a fuel/mixture problem. I'm reliably informed that most injection problems can be laid at the door of air/fuel leaks and bad connections but if there is a computer involved who knows!!!***
Allan Reeling

Roger, Meant to add "sooty" is different from oily of course.
Allan Reeling

Allan,
A long and boring story of an air leak and some stationary running explains the soot - the plugs are cleaner after a long run but the possibility of an air leak is interesting - the cold start stepper valve is the weakest part of the system but the OE rocker covers are ill-fitting and it is possible that the gasket is being sucked into the top of the engine with increased vacuum (under acceleration) but would it close up when the engine cooled ? I will check the plugs as you suggest but I am hoping that as the coolant level is unchanged - there is no steam cleaning. I will also run her on an OE computer with another new HP pump (and filter). More next weekend. Thanks for the advice.
R
Roger W

Roger,
Have a look at the amount of crankcase ventilation there is. I've seen the valley gasket "fluttering" because of insufficient. Also heard of blown oil seals and of venting dip-sticks!! The factory V8's had a filtered vent into the valley, plus negative pressure supplied by the S U's. My conversion has a vent on each rocker cover to the inlet manifold via a catch tank plus a filtered intake on a rocker cover.
Clive Wheatley does excellent rubber rocker gaskets, no ill fitting with them i can assure you.
Allan Reeling

SECOND UPDATE
Code reader showed a fault with the stepper motor or bad idle setup. Reset and cleared it. Back on the highway for re-test, but still not right so checked the plugs for steam-cleaning (all normal) so onto the fuel pumps. More anon.
Roger W

Problem solved.
The main HT lead was tracking onto the expansion tank hose. You wouldn't believe it !!!
Roger W

Hi Roger,

Glad you found and solved your problem.

Sometimes "simple" things are the hardest to find.

Years ago I had a new Morris 1100, and occassionally after driving for a while it started coughing. Couldn't find anything, till one night it started to cough, just as I got home. Lifted the bonnet, leaving the engine coughing, and there was the aurora borealis across the ign leads, those crappy carbon filled ones. A new set of copper leads with suppressors cured that fault.

Herb
Herb Adler

Herb - similar problem years ago with a newish Simca - it was raining and I got drenched.

Mixed feelings this time because our local garage owner discussed all our ideas and immediately singled out the leads and coil. I came straight home and found the tracking spot.
Roger W

This thread was discussed between 17/09/2013 and 06/10/2013

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.