E Class 300TD misfire on cold start

[mebobster]

hi everyone, own an R reg E300TD, was having difficulty starting in december & the glow plug light stayed on. got it tested & found that number 6 glowplug was faulty. pretty obvious really! so, i got all glowplugs replaced. got the car back this monday. now, on a cold start, it misfires for a few seconds only and then engine idles fine! i am totally baffled. faulty glowplugs? or is there something else i should be looking at? any help appreciated. thank-you.

 

[brandwooddixon]

You could have it compression tested, but it sounds to me as if there's a glow plug not working to spec.

Incidentally, if once the engine is warm you try turning it off and restarting it, does it exhibit the same problem?

 

[Cosworth]

possible faulty injector

 

[mebobster]

thanks for reply. when the engine is warm, it starts perfect, the problem only happens on a cold start, for example first thing in the morning, even when i have heated the glow plugs 5 times via ignition. new glowplugs have been fitted, so probably one or more maybe faulty. if i test them with an electrical tester & the tester shows that power is going to all plugs, can a glow plug still be faulty? and is there an easy way of testing injectors? cheers.

 

[Parrot of Doom]

When you start it tomorrow morning look for air bubbles in the fuel lines behind the fuel filter.

 

[dougjoy]

I am having the exact same symptoms, cold start seems to start on 5 for a second or two and then clears. I also get a knock at the same time, (sounds like deisel knock). I have just replaced all my glow plugs as well. I am not getting anything on the dash to say the glow plugs are faulty as before they were changed.
I checked the removed ones and one was faulty, gone high resistance, probably dis. all the others were ok
I have also recently changed the fuel filter and the seal behind the shut off valve. The leak was fixed and the car was starting fine, anyway until the glow plug went. I'm like you and want to go back in and check everything.

You know how you clutch at straws, well when I bought the glow plugs I got 5 from one dealer and the sixth from another, and it was a different coloured metal, everything else was the same. I ' ll try and have a go over the weekend.

Let me know how you get on, and I'll do the same.

 

[mebobster]

Parrot of Doom: thanks, will check for air bubbles in the morning.

Dougjoy: my 97 E300 TD also purring at 118K. interesting to hear that i also get 'diesel knock' on a cold start but i believe that's quite normal? hope so! i had a nighmare getting 6 glowplugs of the same make, it took me 2 days! i changed the fuel filter in may 06 when i bought the car, i may change it again. i will also look into the seal behind the shut off valve, didnt think of that. funny thing is the car runs 100% after a few seconds... ill see how it goes, never had his problem with the car, yes, please do let me know how you get on, i'll do the same. cheers.

 

[sweeper]

Parrot of Doom: thanks, will check for air bubbles in the morning.

Dougjoy: my 97 E300 TD also purring at 118K. interesting to hear that i also get 'diesel knock' on a cold start but i believe that's quite normal? hope so! i had a nighmare getting 6 glowplugs of the same make, it took me 2 days! i changed the fuel filter in may 06 when i bought the car, i may change it again. i will also look into the seal behind the shut off valve, didnt think of that. funny thing is the car runs 100% after a few seconds... ill see how it goes, never had his problem with the car, yes, please do let me know how you get on, i'll do the same. cheers.

Not being funny, but a bit of diesel knock and a slight misfire on cold starting a diesel would be normal, however if you are concerned something is wrong, glow plugs or injectors and cylinder compressions would be the way to go, electronic diesel controls can also take a second or two to settle down, my van for instance, sometimes it will fire and stop, sometimes it will run slightly off for a couple of seconds and sometimes it will hesitate when coming off and on the throttle in first or second gear, been like it since new.

 

[mebobster]

Parrot of Doom: No air bubbles at all!

Sweeper: I appreciate your reply and do understand what you say. Ive had my car since May 2006, & it never had a problem starting in the cold. Around Dec 06, when it was getting colder, it started to misfire on a cold start, & was getting worse, then glow plug lights stayed on for a few mins. Tested glow plug switch & found number 6 had no power, so obviously glow plugs needed replacing. Old glow plugs did look worn, so new ones fitted. Now, the glow plug light doesnt stay on for a few mins, its just normal, but the engine still misfires for a few seconds. Didn't do it before. Not worried about diesel knock, but im not happy with the misfire because it didnt happen previously. Im probably going to end up changing glow plugs again, but im convinced its something else. I shall keep persevering! Just wondered if there was a way of finding out if power to the glow plugs was distributed properly, anyone??? Cheers.

 

[Parrot of Doom]

From what you say, you measured the voltage across the GP and found nothing - this doesn't mean the GP is faulty, it suggests the GP relay is faulty.

Check the GP relay - its a black plastic box on the right wing, big connector on the side of it. It contains 6 fuses, 1 of them may have melted. Heres what they look like:

 

[dougjoy]

Mebobster, is yours still doing it now with the air temp up a bit, my one isn't. I don't know if it is OK until a nice cold morning. I checked the glow plugs and all was ok. What I did was took off the battery for about 20 minutes to see if the engine ECU had a memory unit that learns, and if so to wipe it out. My Honda Prelude has one and if you change plugs etc, to make any difference to the engine it has to learn that things have changed. You do this by wiping out the memory and let the engine tickover until warm,about 15 min and then switch off. The difference in the Honda was very noticable.

I did this with the E300 and it is starting OK but the air temp may be making the difference it is arroung 10C down here this morning.

Until a nice cold one I won't know if this has done anything. I'll keep in touch. I like POD's fuse thing though, I'll check tomorrow.

 

[mebobster]

thanks Parrot of Doom for taking the time to put photos on thread. will be checking this out on my day off tomorrow. cheers.

 

[dougjoy]

Mobster, It is still starting fine, I don't know if you want to try clearing the ECU history. My thinking was that the ECU may have tried to compensate for the faulty glowplug and was still trying to do it once the plug had been replaced.
I was hoping someone would comment on what I've done and say whether it was correct or not.

The only problem with disconnecting the battery is having the radio code handy and resetting the auto part of the windows. Everything else was fine.

I still have not had a very cold morning, but it seems to have cleared.

Any comments

 

[mebobster]

Parrot of Doom: Glow Plug Relay all tested & working ok.

Dougjoy: Thanks for tip. Spent a bit of time on my car today on day off. Eliminated the relay as a fault. Also, the new Glow Plugs tested again and are all fine. Referred to a local Merc specialist by a friend (used to work at local Merc dealer but now has gone independent) who kindly offered free advice. He suggested i change the mini pre fuel filter which is located next to the main fuel filter. Ive done that. He also said that E300 TDs are prone to air leaks in the fuel system. He said that since the car stands overnight, all the fuel goes back into the tank, and first thing in the morning it misfires for a few seconds due to air being present. I did check for bubbles & air leaks in the pipes but couldnt find anything. The car has recently had a new head gasket fitted (due to glow plug snapped in head, not overheating etc) so it has been checked through properly & runs perfect. I dont know if there is another way of testing for air leaks in the fuel, i believe there is some kind of kit available? ill do some more research. What i will do on friday is disconnect the battery totally & hopefully clear any ECU history like you have done, I really hope that works because the car is such a joy to drive 20 seconds after a cold start! will let you know, fingers crossed eh?

 

[dougjoy]

Hope it works for you as it seems to have for me. Is yours still doing it every morning no matter whether the weather is cold or warm. As \I said my one has not done it since the reset, but wanted a nice cold morning to make sure.

Best of luck, for the window reset, open fully with button and keep button down for another 10 sec, then fully up and hold button for 10 sec. Auto up will now work on that window, then you have to do the rest. I had one that still worked so only had to do three.

Let me know how you get on.

 

[Parrot of Doom]

mebobster, when you changed the prefilter did you also put a new o-ring on it? Because they're not supplied with one.

If not, get a new one from the dealer.

If you fill the tank to the brim and then park the car nose down overnight, if that alleviates the problem then you almost certainly have a fuel leak.

 

[mebobster]

dougjoy: left battery disconnected all night to clear ECU history, but it still did the same this morning, naughty merc! it misfires for a few seconds regardless of warm/cold day. shame really. will be trying what Parrot of Doom mentioned tomorrow, which is to fill tank to the brim, & park car nose down overnight. hope that works, if not, god knows! thanks for the electric windows tip, i wouldnt have known that... fingers crossed! i dont think i have a fuel leak though, cant smell any diesel or see any visible leaks, but i have to try it.

 

[Parrot of Doom]

If there is a leak, what will happen is that while the car is parked the diesel will fall back into the tank, and air will be sucked in through the leak. So you won't necessarily smell any diesel.

The reason parking nose-down with a full tank can work is because the diesel in the top of the tank is then higher than the fuel pump, so the diesel cannot fall back, and hence no air is drawn in

 

[dougjoy]

Sorry to hear it didn't work for you, mine is fine now, the only other thing I have done is change the oil, which would not make any difference, and I drove it to Glasgow and back 469 x 2 miles.

Good luck, I hope you find something positive. Parrot of doom will get it, he won't give in until he does.

All the best

 

[mebobster]

hello all. tried what Parrot of Doom suggested. filled tank to the brim, & i mean to the brim! parked car nosedown, jacked up rear of car etc. but couldnt leave it overnight due to the extremely windy weather, but left it like that for about an hour, went to start it, but... it still misfired. was gutted mind! not giving up yet. also discussed with others, & i am sticking with the 'air in the system' problem. double checked all pipes again, but i am sure the engine is sucking in a bit of air during a cold start (after 5-6 hour rest) from somewhere, hence the misfire. i think we have covered a lot in this thread, im just dead jealous of dougjoy!!! now i have to move onto confirming an air leak somewhere. ive been told that there is a kit availale to check for air leaks, is there? im never going to give up on this one, believe me, i just want to avoid booking it into a merc garage cos i know i can do it. never lose faith eh?!