Thread: 300tdi Misfire

Hi guys and gals,

This is my first post and I'm hoping someone can help me with a problem. First of all I have a 96 300tdi Disco, new head 6 months ago, timing belt done 15000km ago. New 2.5" exhaust a few weeks ago and new K&N Air Filter at the same time. Was running perfectly

I ran my tank dry on the way to the servo about a week ago, managed to get to the pump but it had sucked in a little air, started ok with the tank topped up though and ran fine for a couple of days. Now it has a misfire, not noticable when you first start it and it idles perfectly but when I drive it it seems a little down on power, and after a minute or so it loses a lot of power and misfires, sounds like a single cylinder. It gets quite lumpy but if i let it idle for 10 secs or so it clears up again. Its particularly bad on the highway as its constantly under load and it can buck and shunt around a bit as it misses. If i drop from 5th to 4th gear and gun it it seems to clear up a bit but not all the way. It does seem to smoke more when its misfiring aswell and it misses worst around 1500-2000 rpm.

So far I have:
-Adjusted tappets
-Replaced fuel filter
-Cleaned out sedimenter
-Replaced lift pump
-Replaced the fuel lines from the lift pump to the filter and from the filter to the injection pump
-Used compressed air to blow through the feed and return lines from the engine bay to the fuel tank
-Visual check of the injectors, all looked sooty but not damaged
-Removed fuel sender unit from fuel tank and checked it out. There is wear on the metal tube from the spring loaded mechanism moving back and forth but it doesn't go through the tube.
-I just topped up the tank again to check if it was due to fuel being lower in tank (1/2) but its full now and still doing the same thing

I'm guessing it has to be something to do with fuel supply, but there is a light tappety noise coming from the engine somewhere. If i take the oil cap off some smoke comes out of the engine while its idling. Stationary it will rev all the way till the governor kicks in and has no noticeable miss Has anyone had this same thing happen to them, I'm tearing my hair out here

Originally Posted by bee utey

I was going to suggest a broken valve spring but thought, naah, don't know nuffin about diesels. Can you replace the valve spring in situ? Used to do this all the time on V8's with compressed air. Some cast iron engines with valve seat recession I would fit a washer under the spring to raise the tension, enables a bloke to keep going until he could afford a head job.

Just make a lever to depress the valve retainer with the engine at TDC, see how far the valve goes down.

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The valves are very tough. I know because a real dog of a Discovery 300TDi that I bought had valve imprints in all of the pistons. Presumably the previous owners had got a clown to replace the timing belt! Down in the sump there were lots of broken valve-caps. I made a pretty good job of repairing the engine and fitted new standard pistons after doing a ridge-ream and hone. A new made in South Africa head was fitted as well as a new radiator and timing belt kit. After all that work it pulled like a train but the real bummer happened a few months later at the annual MOT inspection.

"OH I DON'T LIKE THESE, THEY RUST A LOT!" was the MOT testers opening gambit. The tester and his assistant then hammered my truck with a proper 16 oz hammer, NOT the VOSA approved "toffee hammer" and they moaned about rust and the previous owners bad welding. Now I'd bought a boat on eBay and it was stuck in what I call "Shylocks Marina" (they are very expensive and use the same MO as greedy money-grabbing dockers) With the benefit of hindsight I should have chanced the fine and removed the boat with my truck with no MOT. (Brakes, lights, steering, horn, tyres, seat-belts and emissions were all OK) Now I did buy a top class MIG welder to fix it but with the coldest winter and spring for fifty years the machine is still in its box!

Next a Mercedes 300 Touring Diesel Estate was bought (legal towing capacity about two tons) It was spares-or-repair with the declared fault of the alternator not charging. The alternator only took a few minutes to fix as on those old Bosch alternators the carbon brushes and the voltage regulator are a single unit that is held in with two screws. The car was driven home which was 130 miles but I did not find the car a pleasure to drive as my left leg had nothing to do! Later it was found that coolant was missing from the header tank yet there had been no overheating on the journey home. The car had been parked and had not been used since that one trip. Much later it was noted that the oil level was now an inch and a half above the "High" mark on the dipstick. Evidently the coolant is now in the sump so its now dead car #2.

A bit of rust or bad welding can seriously harm ones finances over here. The other aspect is that test stations like to give out a high percentage of failures, well over 50%. One can surmise that they worry that if they pass too many cars the first time, VOSA will come knocking and their testers licence might be cancelled.