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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 20396
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My Octavia 1.9Tdi has been intermittently stopping while driving.

Resolved Question:

My Octavia 1.9Tdi has been intermittently stopping while driving. When it occurs if I get up to speed it is fine but if I slow down it misses and loses power again. When it stops it is very hard to start again. It just clears itself after a while and is perfect for a few weeks. This morning it failed to start. It fired a few times and ran for a few seconds then wont go again. I tested the fuel pressure pump and it is working fine. Took off supply to fuel filter and it pumps diesel when ignition switch turned on. Changed fuel filter as a precaution. What can I check to diagnose fault ? Thanks ! David Dunne.
Submitted: 10 months ago.
Category: Car
Expert:  Matt replied 10 months ago.
Hello This could be a wiring issue as things tend to expand when hot and its possible a poor or partial electrical connection is being pushed apart on one of the major sensors so check the connection plug and wiring to the following;Airflow meterCrank position sensorCam position sensorO2 sensorThrottle pedal position sensor Also worth checking the connections to the fuel pump for the same reason - unplug each in turn and give them a good squirt of switch / contact cleaner - Not WD40 and also unplug and inspect the main power relays and fuses for signs of overheating or corroded terminals - so the ones for the ECU and the fuel pump And also unplug the wiring loom connections in the engine bay one at time and inspect for corroded or recessed pins.
Customer: replied 10 months ago.
Thanks Matt,
I was actually a mechanic for 15 years. I gave up in 1999 and started working as a fitter on machine maintenance instead so I do know my way around engines pretty well. (or I did before they went too computerised) I would have thought that if a sensor was at fault or a connection somewhere that the engine management would pick up on it and give an alert of some sort. I have checked all connections, plugs etc and everything is exceptionally clean and as new. I convinced myself early on that it was a fuel starvation problem hence my check and elimination of the electric fuel pump as a potential cause. I have since discovered that the electric pump simply primes a camshaft driven high pressure pump which also provides vacuum for the brake servo. I have been checking into these "tandem" pumps as they seem to be called and am wondering if they could cause similar symptoms. They appear to have a reputation for causing trouble by leaking allowing diesel/engine oil to mix but do you know if they can cause a problem like mine ? I am still leaning towards fuel starvation of some sort as the possible cause. Basically the car is going fine for weeks. I come out one morning and it is very hard to start. By continuously using the starter I eventually get it going. I have to keep up the revs to stop it from stalling so when I drive it hard it seems fine. Back in my day the injector pump drew fuel up from the tank and it was so easy to see the problem. On my Skoda apparently the injectors create the injection pressure using cam driven rockers of some sort for each injector. I had a theory that if the engine got going fast enough the injectors might do most of the pumping without need for the high pressure tandem pump.
Expert:  Matt replied 10 months ago.
you've understood how it all works, this engine is known as a 'pump duse' type where each injector features its own high pressure pump and is fed by the electric pump
but as you have replaced the pump and filter I'd move onto checking if the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,
Could also be an injector leaking fuel back through the overspill pipe, so not getting enough fuel pressure to tell ECU to fire the injectors, this is trickier to find and you’ll need the garage to do a ‘run back’ test to identify which one is the issue
If the EGR valve is stuck open then this too can effect starting so try disconnecting it to see if this helps or changes anything
Expert:  Matt replied 10 months ago.
do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. so if my answers have been helpful please rate my answer.
Customer: replied 10 months ago.
Been busy over Christmas but my problem hasnt gone away although there has been a breakthrough. I managed to grt the car going by continuously operating the starter. I discovered something that happens when it started. First I was operating the starter with no attempt to start, then a warning sounded around the dash. Immediately afterwards, the engine started but the engine management light stayed on, the oil and heater plug warning light were flashing and the tacho was not working. The car ran and drove ok so I brought it up to temperature and switched off. I immediately tried to restart and same thing occurred. Engine turned over on starter with no attempt to start followed by audible warning then engine started immediately afterwards as before with lights on and no tacho as before. However since then it has been starting normally but this time the engine management light only stays on so finally I have something concrete to follow up. It is obviously in get you home mode as it is hesitant all the time. I am going to get hold of a fault code reader to check this out.
Expert:  Matt replied 10 months ago.
hello I'd check that you're getting full power to the glow-plugs, they pull a lot of current so take care when checking but see if the relay is clicking when the ignition is turned on first and also check the fuse if this is OK check to see if the engine earth strap is also OK.If the plugs haven't been changed in the last 20K then they may have to be replaced.You can test the glow-plugs with a multimeter by measuring the resistance across the plug (between the power connection and the body) it should be around 4-8ohms if its tending to infinity / open circuit the plug is faulty or by using a clip on current clamp, either on the supply wire to glow plugs or onto engine earth, with glow plug light on you should see 15-20amps for each glow plug fitted, 4 cylinders would be approximately 60-80amps.
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 20396
Experience: BEng hons Mech engineering, in auto industry 22 years
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