Engine backfire after running out of petrol

stuthegeordie · Jul 14, 2015

Sender gauge is apparently innacurate resulting in running out of petrol today, put new fuel in and it now runs rough, loads of small backfires, any idea?

Cheers in advance

boguing · Jul 14, 2015

If it really did run out, then best guess is dirt/water sucked up from the dregs. New filter and carb cleanout required.

stuthegeordie · Jul 14, 2015

Thanks for the response. Yeah it ran out, its a new tank so I doubt its crud in the filter. It just burbles away, popping and farting. Im thinking theres air in the system somewhwere.

Wimblowdriver · Jul 14, 2015

Could be that the spark plugs have overheated = damaged due to running on a very weak mixture just before the fuel ran out. Easy to check, take them out and inspect for signs of overheating (damaged insulators, white electrode tips). Or swap for known good ones.

boguing · Jul 14, 2015

What engine?

The reason I said 'if' on the running out was wondering if there was an air leak low down in the supply which gets worse with low level in the tank. Did you fit new pipes and filter when you changed the tank? Or has something been disturbed and cracked?

stuthegeordie · Jul 15, 2015

2.5 petrol

No just replaced the tank to get it through MOT, lines were replaced a couple of years back by PO. It ran like a dream prior to running out of fuel so the impact of it running out has had some impact somehow.

stuthegeordie · Jul 15, 2015

Wimblowdriver

Good call on the plugs .They need changing anyway.

boguing · Jul 15, 2015

If the plugs don't fix it (and I'll have learned something if a moment's weak running can damage a plug) I think a carb strip is in order. P'raps all that pumping to get it going has split the accel pump diaphragm?

stuthegeordie · Aug 7, 2015

Got a chance to look at the Landy.
New plugs and rotar arm have made no effect and the accel pump diaphram looks ok. Air filter is good and it's getting a good jet of fuel. Annoying cos she was running mighty fine prior.

Wimblowdriver · Aug 8, 2015

stuthegeordie said:
Got a chance to look at the Landy.
New plugs and rotar arm have made no effect and the accel pump diaphram looks ok. Air filter is good and it's getting a good jet of fuel. Annoying cos she was running mighty fine prior.

Have you checked for air leaks at the inlet manifold. Not familiar with your engine, but possible leak on the hose between the inlet manifold and the brake servo. If the intake is getting extra air it will cause backfires due to a weak fuel/air mixture. If that is all ok, check the exhaust for leaks, could be sucking air in on the overrun which will cause banging in the exhaust. Do these easy checks before looking at things like the ignition coil and its wiring. Is your engine fuel injected or carburettor?

stuthegeordie · Aug 9, 2015

Wimblowdriver said:
Have you checked for air leaks at the inlet manifold. Not familiar with your engine, but possible leak on the hose between the inlet manifold and the brake servo. If the intake is getting extra air it will cause backfires due to a weak fuel/air mixture. If that is all ok, check the exhaust for leaks, could be sucking air in on the overrun which will cause banging in the exhaust. Do these easy checks before looking at things like the ignition coil and its wiring. Is your engine fuel injected or carburettor?

Thanks Wimblowdriver. Its a 2.5 with a webber carb

stuthegeordie · Sep 25, 2015

Think I've finally figured it out!! Found a leak at the joint at exhaust end piece, fixed it thinking all was well. Of course not, further investigation and I discover a crack in the exhaust manifold around the stud. All this due to running out of fuel or a massive coincidence?

Brown · Sep 26, 2015

Coincidence! I find it a bit difficult to imagine how running out of petrol can crack a manifold. If that was true we'd need to replace manifolds like we do with air filters. Even if it might get a wee bit hot with running on a weak mixture before it finally runs out.

stuthegeordie · Jan 25, 2016

Brown said:
Coincidence! I find it a bit difficult to imagine how running out of petrol can crack a manifold. If that was true we'd need to replace manifolds like we do with air filters. Even if it might get a wee bit hot with running on a weak mixture before it finally runs out.

Turns out exactly that, took it to a Landy mechanic he recons when it was running out of fuel it would be running lean and the additional heat would be enough to crack the 30 year old manifold.

boguing · Jan 25, 2016

Not sure that I can believe that. In any case, a cracked exhaust manifold won't cause a misfire.

waldamar · Jan 25, 2016

Fuel pump weak? Often overlooked - and a static test doesn't let you know how it'll behave under load.............

Wimblowdriver · Jan 25, 2016

boguing said:
Not sure that I can believe that. In any case, a cracked exhaust manifold won't cause a misfire.

Correct, it won't cause a misfire, but it will cause banging in the exhaust especially on the overrun when the vacuum in the exhaust system sucks air in through the crack.

waldamar · Jan 25, 2016

Just to explain fuel pump theory - pump is weak so can't keep float chamber full - hence spit and pop just like running out of petrol, then pump catches up, engine runs then process repeats itself. If you've got a hand lever/prime lever on the pump give it a minute or so of pumping then start her up - possibility all will be fine for a few moments or you rev it up a bit - then she'll probably start missing again - worth a try, good luck

stuthegeordie · Jan 27, 2016

The popping, it turns out was due to the crack in the manifold sucking air and creating the pop.

waldamar · Jan 27, 2016

stuthegeordie said:
The popping, it turns out was due to the crack in the manifold sucking air and creating the pop.

Excellent - all sorted then? New manifold?

Engine backfire after running out of petrol

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