Very Strange Running Problem

[klittle]

Well I have scratched my head long enough so hopefully one of you here can perhaps shed some light on the issue i'm having, this is as much detail as I can think of :-

MK2 1.8 with Weber carb & free flow filter, tope end overhauled last year with new head gasket.....

Car warms up & runs fine for first 40mins or so with temperature sitting just above the half way mark, then the temperature starts to drop down & it misfires, so much so that I have to do left foot braking to keep the throttle going or it will stall. If I put the foot down to open second stage on the carb it will pick up, Fuel system is perfect with no issues as it was all fully checked the end of last year & is mint, definately not a fueling issue, I at first thought perhaps carb icing, but no, as the issue happens no matter the air temp. It's the fact that the temp gauge drops to that's got me, i'm wondering if it's head gasket related, but it's not using water & no white residue on filler cap, so perhaps between 2 cylinders (again). On top of that, if I pull up & keep it idling for a few mins the problem clears & it will idle fine, although by that time I have got to work & I'm in the carpark. Oh, the heater in the car shows no sign of blowing any cooler when the temp gauge drops either. It is using oil as I suspect one of the new stem seals has damage.

I do have another engine sitting, bog standard 1.8 carb again but lower mileage, but just wanted to run this one past everyone to see if you had any thoughts.

Just to confirm, it's not dirt in the fuel, no loss of coolant, does use oil but not blowing smoke all the time, just on changedown and heavy acceleration, carb has been overhauled and full ignition system has been replaced. Starts on the button every time other when the misfire issue arrises, then it's fine when it's cooled down.

Over to the experts.......please!!!

[haddock_n_chips]

Interesting symptoms..............

my initial thought is (this is coming from a classic car background, and many years Rocco ownership)

I would go back to think of fuelling issues. I know you had it set up etc... and running sweet but the symptoms sound familiar to fuel starvation caused by fuel evaporation

i.e. fuel evaporating due to heat before it can get into the carb

Shoving the throttle down forcing more fuel through thus compensating for the problem.

Its just a thought but perhaps this could be the case?

Its a common problem i have encountered myself on MG's, particularly when running non standard equip

I drove an MGB GT v8 last weekend and had a similar problem, car runs sweet as a nut until fully warm then cuts out unezplicably once hot.

Fuel evaporation before hitting the carb, engine splutters and dies...... bastard to start again too without flooding the carb!

this however wouldnt really expain the drop in temperature unless the fan is kicking in, thermostat opening and the cooling system doing its job, could be unrelated....

hmm.......

anyone else with any ideas?

[Neillyboy]

I'd check the rubber flange between the carb and the inlet manifold. Run the car and give the carb a wiggle about and you'll hear it drawing air or a change in note if there is an issue without it being visible. I don't thing the temperature drop is related but I could be wrong.

[klittle]

Thanks for the replies and I can see your thinking. Fuel evaporation sounds right for the misfire fault, but I think it's being caused by the temperature issue. My thinking is this, I suspect the water pump is damaged internally or partly blocked, when the engine heats up the waterway at the pump is being restricted causing an air lock or lack of flow through the head and with less water passing the sensor it would show a drop in temperature, no other way for it not to pick up a signal. This heat is causing the manifold to over heat and evaporate the fuel. Hopefully you follow my thinking so far lol. Anyway, I plan to change the water pump thus weekend as I have spare car sitting and it's not difficult. That way the system gets a flush and I can see what the old pump looks like for anything obvious. Will let you know how I get on, but I know fuel system and seals are good as as just checked them and all good.

[klittle]

Update - I changed over the complete pump yesterday, still need to take it for a drive, but idling in drive it seems fine, temp comes up to just above half and then fan kicks in and keeps it stead around the half mark, which looks better. The pump I removed looks pretty new and spins OK, just a bit on the stiff side, which I was told last night can be normal for a new pump. What my mechanic guy did tell me was this, the thermostat could have been sticking and may have been the cause of the problems, I have left the thermostat that was fitted to the spare pump on it and will check the other one. I can see his thinking, but only time and a few tests will tell.

[Tim_R]

You could pop the thermostat and a suitable thermometer into a pan of water on the hob and bring it up to temp to check it opens and closes when it should and as much as it should. The opening temp should be marked on the thermostat.

[klittle]

That's exactly what I plan to do, but I have since taken the car for a run and it's completely gone the other way, temp sitting while driving and about a quarter, but it is up to temp as thermostat open, comes up to half before fan kicks in when sitting still. I'm relatively happy with it now, I fully expect the gauge to show colder when driving as its darn cold out today. Time will tell if any damage done by other thermostat, but nothing showing so far, fingers crossed.

[Old Mac Rocco]

My wee 1.6 never goes past the half way mark if it is going to the shops or a run to Dunfermline or Leeds.
I think I once hear the fan kick in about four years ago but I do check it occasionally to make sure it will kick in if required.
I remember when it kicked in as it was one of these "never heard that noise before moments" and pulled up as soon as I heard it.
The needle very seldom touches the warning light and normally sits just to the left of it.

[klittle]

Well the fault continues, thermostat is Ok and so is fuel and ignition system. Seems to be OK till it's been running a while and then it's sucking in oil either from valve stem or past piston rings or both, then it shoots up and misfires. I feel an engine change coming on. Spare is OK and I'll strip the one I take out and do a but and bolt rebuild, including head and block skim, some porting and general balancing. Will make up some pics as I go once I start. In the mean time it can run as is or go bang, really not worried about it.

[Old Mac Rocco]

Sorry to read that it's not sorted itself out.
Pity you weren't a bit closer or some of us could have given you a hand but it sounds as if you have everything covered.
Nearest help to you is DT1 but this is his busy time plus he has not been on much lately.
If you are looking for any bits and bobs give us a shout and we'll raid our stores and see what we have.
Good Luck.

[wassell]

Is it a weber or pierburg? Wondering if a dodgy temp sender is making it choke when not needed? causing issues

[bengould]

Is it a weber or pierburg? Wondering if a dodgy temp sender is making it choke when not needed? causing issues

1st post!?!?

[klittle]

Thanks everyone, but I do have a VW expert as a friend and he has offered his help......it's a Weber carb so no temp sensors involved, it's def sucking oil and sooting up plugs, hey ho, it's all good fun and I have the sheds to work in luckily.

[klittle]

Oh well, think I got to the root cause, looks like a soft seize on the engine caused by lean mixture. Basically running too hot and melting piston rings. When hot it was sucking oil past the rings and sooting up with lack if fuel. I can't prove anything but think garage played with it when trying to get timing and running set up. Mixture screw now sitting at 2 turns out and it's at least driveable till I get the new KR engine.

[klittle]

Oh well, think I got to the root cause, looks like a soft seize on the engine caused by lean mixture. Basically running too hot and melting piston rings. When hot it was sucking oil past the rings and sooting up with lack if fuel. I can't prove anything but think garage played with it when trying to get timing and running set up. Mixture screw now sitting at 2 turns out and it's at least driveable till I get the new KR engine.