Broken down - no engine power

Post Sat Apr 11, 2015 11:34 am

Currently stranded. Engine starts fine but idles low. If you press the throttle it blips up to 3000 rpm but immediately drops back to idle, no matter how hard you press the pedal. It threatens to stall if you try and move, even in first gear.

Symptoms feel like an air leak or a dead AFM, but I cant find any issues in the engine bay. I've sent the wife off for fuel (even though the gauge reads 20 litres), but does anyone have any other ideas?

Post Sat Apr 11, 2015 11:48 am

Must be fuel related, maybe a slight blockage, with the short 3000rpm blip being the fuel rail depressurising.

Post Sat Apr 11, 2015 12:57 pm

Put 5 litres in, no improvement. Managed to limp it home in 1st/2nd gear, but now clueless what to check.

Post Sat Apr 11, 2015 1:20 pm

Grrrmachine wrote:Put 5 litres in, no improvement. Managed to limp it home in 1st/2nd gear, but now clueless what to check.

The wiki?

Post Sat Apr 11, 2015 1:22 pm

Very droll malc.

Post Sat Apr 11, 2015 1:23 pm

Have you checked that the AFM flap can/is moving freely?
jimbom30cab's fuel pressure idea might be worth checking out. Restricted fuel supply can give some odd symtoms.

Post Sat Apr 11, 2015 1:23 pm

On a more serious note,I would start with cambelt timing,your lack of power suggests that the inlet valves are opening late.

The above assumes that you have already checked for a blocked fuel filter,and that fuel is being delivered at the correct pressure.

Post Sat Apr 11, 2015 1:42 pm

Now that the car's back in the garage I can be a bit more calm and methodical about troubleshooting. But since I'm not going to back to that hateful machine today, I'll give you a bit more backstory.

I worked on the car last weekend and it was fine, but this involved disconnecting the battery. I reconnected it this morning to a stone cold engine, did the 10-minute (ok, 5 minute) warm up ECU learning procedure, then set off. The exhaust stank of fuel during the idle procedure, but I assume that's just the ECU getting the mixture wrong to start with.

I drove about 10 miles, and it was very rev-happy and pulling strong, and had no problem with either fast roads or traffic. Stopped at a shop, then set off again, and got stuck in lots of idling traffic. As soon as the traffic flow increased again, the car lost all power and I rolled it to a stop, but with the engine still idling. I didn't notice any issues on the cluster - no overheating, no fuel light, no oil pressure warning - so I'm hoping I haven't done something terminal like broken a rocker arm.

Stuck fresh fuel in it but no improvement. If you turn the key it starts immediately and idles at 550-600 rpm (too low). Press the pedal in any way and it'll rev up but then immediately drop back down to 500rpm, and then it'll choke and splutter, occasionally revving back up, until you let your foot off the pedal. I drove it home 5 miles like that, and it didn't overheat or have any oil pressure issues, but the exhaust still stank of fuel when I put it in the garage. The top speed on the journey home was about 25mph, but most of it was 5-10mph spluttering.

I haven't done more than a cursory check of the engine bay (too hot, too stressed out), but I couldn't see any loose air hoses and the HT leads were all on tight. It also wasn't dribbling any fluids on the floor during idle, so I don't think anything's burst.

Post Sat Apr 11, 2015 1:51 pm

pull a plug out see how the tip looks, probably sooty ? i had a similar issue once, IIRC it was TPS related

Post Mon Apr 27, 2015 9:23 am

Thought I'd update this.

Went for a long drive this weekend and suddenly developed a misfire - engine was popping at odd moments, and even when it settled down it wouldn't rev over 4000rpm. Pulled over to a service station and wiggled the HT leads to see if they were alright. Pulled away and lost all power - exactly the same symptoms as during the original breakdown.

So popped the bonnet again, disconnected then refitted each HT lead at the spark plug, and she fired up again and drove on - still with a little misfire, but definitely improved.

I need to get some HT leads on order now - anyone got any recommendations? The current set are Bremi, of unknown provenance (they came with the engine 3 years ago).

Post Mon Apr 27, 2015 10:00 am

Grrrmachine wrote: anyone got any recommendations?

Genuine, 30 year old leads, from an unmolested E30 breaker.

Post Mon Apr 27, 2015 2:00 pm

I thought you'd say that Brian, but surely even 30-year-old ones are starting to get a bit ropey by now. I do have a spare set from my original M20, but thought a new set might be a wiser purchase.

Post Mon Apr 27, 2015 6:46 pm

i bought a set of new leads from ecp for about £180 original bremi ones

Post Mon Apr 27, 2015 9:04 pm

WillNZ wrote:OEM leads are copper cored

Copper cored, plus top quality type of high voltage series resistors.
If there's no green gunge inside the caps, and the rubber isn't a mass of cracks when you bend the leads, they are good to go.

Post Mon Apr 27, 2015 9:26 pm

First thing I thought was cap with a crack and second was leads. Not often will these lumbes have internal issues and not spill.

Post Tue Apr 28, 2015 5:56 am

I'll be doing a full service this weekend, so pulling each spark plug will give me an idea of where the problem is, since it's clearly an ignition issue.

The distributor cap was replaced 6000 miles ago, but I'll check the rotor just in case it's started arcing badly.

Post Tue Apr 28, 2015 9:19 am

Failure of one individual plug lead on a M20 will barely make a noticeable difference to it's running. To have the kind of effect you are describing, the problem would have to be in the coil - king lead - distributor cap area of things.
Definitely check the distributor cap/rotor, and pay attention to the sprung carbon button in the centre of the cap.
Chief suspect is always the last part to have been replaced.