Engine not running under load
Discussion
Hello,
Strange problem this week with my car (1989 Mercedes 300SL) after cranking for ages it will start and run perfectly even with some revs, however when I engage drive it will cut out before moving any distance.
First thought was a blockage in the fuel filter so changed this but no joy.
Other info is
-No smell of fuel from the exhaust after lengthy cranking
-After cranking on releasing the starter engine sort of runs to standstill, i.e. sounds like it has started but quietly
-Battery is charged and cranking is strong
Think that is all the information, can anyone help ?
Thanks
Ben
Strange problem this week with my car (1989 Mercedes 300SL) after cranking for ages it will start and run perfectly even with some revs, however when I engage drive it will cut out before moving any distance.
First thought was a blockage in the fuel filter so changed this but no joy.
Other info is
-No smell of fuel from the exhaust after lengthy cranking
-After cranking on releasing the starter engine sort of runs to standstill, i.e. sounds like it has started but quietly
-Battery is charged and cranking is strong
Think that is all the information, can anyone help ?
Thanks
Ben
Any idea what sort of fuel system you have? It sounds a bit like the symptoms you'd get on an injection setup with no / minimal fuel pressure. It'll suck fuel in due to manifold depression, but there isn't much of that when cranking or when you load the engine. Of course in this case it'd also die when you rev it.
Sounds like the fuel pump's not doing it's job, or for some other reason, fuel isn't getting pressurised in the fuel rail. Does the fuel pump prime the rail when you first turn the ignition on? I had a car do this once, it would prime the rail, so it would start and run for a few moments, but would then just die away.
PS, you do have fuel in it don't you? Wouldn't be the first time......
PS, you do have fuel in it don't you? Wouldn't be the first time......
At its most basic an engine only needs three things to run. Compression, spark and fuel and all working at the correct times. So start with the basics. Compression test to make sure the engine internals are still ok. Then check the spark at each plug and if possible the spark timing with a strobe.
Finally check fuel system pressure or pump output and that no vacuum or other hoses around the engine have split causing an air leak.
If nothing shows up you'll then have to start looking at things like temperature sensor resistances and any other sensors like throttle or crank position ones if applicable.
If I had to guess though it sounds to me like lack of fuel or weak mixture from an air leak.
Dave Baker
Finally check fuel system pressure or pump output and that no vacuum or other hoses around the engine have split causing an air leak.
If nothing shows up you'll then have to start looking at things like temperature sensor resistances and any other sensors like throttle or crank position ones if applicable.
If I had to guess though it sounds to me like lack of fuel or weak mixture from an air leak.
Dave Baker
Snake the Sniper said:
Sounds like the fuel pump's not doing it's job, or for some other reason, fuel isn't getting pressurised in the fuel rail. Does the fuel pump prime the rail when you first turn the ignition on? I had a car do this once, it would prime the rail, so it would start and run for a few moments, but would then just die away.
PS, you do have fuel in it don't you? Wouldn't be the first time......
Plenty of fuel in the tank (and I know I would not be the first)PS, you do have fuel in it don't you? Wouldn't be the first time......
I can hear the pump prime, off out in a moment to confirm it is actually pumping
To answer another questions it is a daily driver so is used a lot
Symptoms before the problem occured were a reluctance to rev, this happened about a mile from home and it made it all the way, at the time I thought a quick Italian tune up would clear any blockage, sadly not the case.
Ben
Some progress, I have been able (in between rain and work) to remove the fuel feed to the fuel distributor and confirm that fuel is being pumped. Have also confirmed a spark. Therefore the problem is probably in the fuel distributor (Bosch KE Jetronic uses a funny fuel distributor type thing) Will look at that this eve
Ben
Ben
You've been caught out by two of my favourite rules about engines which are.
1) It's nearly always something basic rather than the most complex thing you can conceive of.
2) Never assume something you changed recently is ok and not worth looking at.
I borrowed an old VW Polo a few years ago from a mate while I was looking for a new car. It ran ok for a couple of hundred miles and then started cutting out every few miles but restarting again if I left it for a few minutes. It would then run for a few more miles and stop again, usually in the worst possible place like the fast lane of a motorway. First step in the logical chain of diagnosis was to ask what its history was. I was told it had sat for ages but then been serviced before being put back into use including a new fuel filter. Ok so it can't possibly be the fuel filter.
So I started working my way through every other thing, including coil, leads, plugs, full strip and clean of the carb and several weeks later had got nowhere and it was still doing it. Finally it conked out on a country road one day and I happened to notice the fuel filter was full of cloudy petrol and clearly blocked solid.
I got a new one and it ran ok for a while and then started doing it again. Sure enough the filter was blocked again. Cleaned the old one out and swapped them and it was fine. I had to do this several more times to finally cure it but what had actually happened was it had sat so long the petrol tank had gone rusty inside and the flakes of rust were blocking the filter. As I drove along these would stir up and block the entire element and after it had sat for a while they'd settle to the bottom of the filter and let it run a bit longer. Eventually I worked them all out of the system by changing the filter every few days for the old cleaned one.
If I hadn't asked what had been changed recently I'd have gone to the filter straight away. C'est la vie. My own rules should still have told me it was likely to be one of the basics though.
On cars with distributers that won't run properly it's ALWAYS the cap at fault not the plugs or leads and hardly ever the coil or rotor arm. Failing leads tend to start displaying problems in wet weather first and it's very rare that plugs will stop an engine running without it being obvious from their condition. Coils can be checked easily with an ohm meter and if the readings are ok then so is the coil. Distributer caps are little b
ds though. They don't always show tracking marks inside even after they've failed. Always change them first if an engine starts playing up.
Dave Baker
1) It's nearly always something basic rather than the most complex thing you can conceive of.
2) Never assume something you changed recently is ok and not worth looking at.
I borrowed an old VW Polo a few years ago from a mate while I was looking for a new car. It ran ok for a couple of hundred miles and then started cutting out every few miles but restarting again if I left it for a few minutes. It would then run for a few more miles and stop again, usually in the worst possible place like the fast lane of a motorway. First step in the logical chain of diagnosis was to ask what its history was. I was told it had sat for ages but then been serviced before being put back into use including a new fuel filter. Ok so it can't possibly be the fuel filter.
So I started working my way through every other thing, including coil, leads, plugs, full strip and clean of the carb and several weeks later had got nowhere and it was still doing it. Finally it conked out on a country road one day and I happened to notice the fuel filter was full of cloudy petrol and clearly blocked solid.
I got a new one and it ran ok for a while and then started doing it again. Sure enough the filter was blocked again. Cleaned the old one out and swapped them and it was fine. I had to do this several more times to finally cure it but what had actually happened was it had sat so long the petrol tank had gone rusty inside and the flakes of rust were blocking the filter. As I drove along these would stir up and block the entire element and after it had sat for a while they'd settle to the bottom of the filter and let it run a bit longer. Eventually I worked them all out of the system by changing the filter every few days for the old cleaned one.
If I hadn't asked what had been changed recently I'd have gone to the filter straight away. C'est la vie. My own rules should still have told me it was likely to be one of the basics though.
On cars with distributers that won't run properly it's ALWAYS the cap at fault not the plugs or leads and hardly ever the coil or rotor arm. Failing leads tend to start displaying problems in wet weather first and it's very rare that plugs will stop an engine running without it being obvious from their condition. Coils can be checked easily with an ohm meter and if the readings are ok then so is the coil. Distributer caps are little b
ds though. They don't always show tracking marks inside even after they've failed. Always change them first if an engine starts playing up.Dave Baker
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff