Car problem - misfire?

digitalscream

So...having replaced the front calipers/disks/pads, exhaust and rear bumper, I only have one remaining problem with the ZR and it's a bit of an odd one.

Under normal conditions - eg taking the sproglet to work - it's absolutely fine. Starts first time every time, idles smooth, runs well and has plenty of pickup. The problem occurs when you stop for a bit and turn the ignition off. On firing it up again it seems to idle OK, but pulling away there's almost no power and when it goes back to idle it's rough as hell and the engine is rattling quite a bit. Flooring the accelerator does absolutely nothing, it stays in "rattly idle" mode. The only way to get it back to normal is to give it some revs (~3.5 - 4.5k) with the clutch down in first gear and then let the clutch up pretty quickly...which can be a bit disconcerting for passengers and other road users alike. Two or three times it's actually backfired while doing this, although that hasn't happened since I put the new exhaust on.

The odd part is that two or three times it's done it when just using the handbrake at a junction (and once at a pretty fast-moving roundabout, which was scary as hell). The front brakes were seized on when I got it (hence replacing the whole lot), and I sort of wonder if it's possible the rear brakes occasionally do the same; visual inspection suggests they probably need replacing too, but...priorities.

Any ideas? It seems to fit with the idea of dodgy spark plugs or HT leads, but I'd have thought that wouldn't be such an intermittent problem.

guitarcookie1

Do you have a local friendly garage you could take it to and ask them to plug it into a fault-code reader?

This should trace the problem quite quickly if it's something like an oxygen sensor?

Funkfingers

+1 for fuel regulation and/or ignition electronic problem diagnosis. Failing coil pack could explain it.

ICBM

That sounds very like a problem I had with my old Citroen Xantia - it was never traced conclusively but my garage chap was almost certain it was a fault in the electronic engine management unit. (Quite likely just something like a cracked solder joint inside, but it's a sealed unit and not designed to be worked on.) The only way to be sure would have been to replace it and see if the fault went away. Given the cost of even a second-hand unit (which could easily have a similar fault, since it was very intermittent and might well test OK out of the car) I traded the car in for a new one, for basically the scrap value...

FX_Munkee

As suggested above a failing coil pack often exhibits stuff like this as they tend to fail as they heat up, resulting in poor warm starts and fluctuating idle speed (as ECU tries to compensate with fuel). HT leads also break down as they get hot.
The back fire would indicate it's spark related as it would be because there's unburnt fuel in the exhaust.
It could be Mass Air Flow sensor (MAF), but I'd be looking at the coil and HT leads first, mainly because it only does it after heating up a bit.

tone1

Has it got coil a black distributor cap or has it got 2 coils and 2 HT leads under the top engine cover that is held on with 2 Phillips head bolts?
if it's got distributor cap and coil down the back of engine block then it's probably rotor arm going short circuit although coils down the back of block are common fault...

if if it's the type where you have a coil on 2 Cylinders and 2 small HT leads leading to the other cylinders then try inspecting the leads for burning/arcing

FX_Munkee

Please tell me Rover weren't still making cars with distributors that late on, were they?

digitalscream

FX_Munkee said:

Please tell me Rover weren't still making cars with distributors that late on, were they?

Nope - two coil packs. Each sits on a spark plug, with an HT lead to the other spark plug.

FX_Munkee

Yes, I'd be looking at those. TBH if they're not very expensive I'd just change them as a way of eliminating them as a possiblity. That and it's an easy job you can do yourself.

tone1

I'd start with the 2 leads first...clean them and the track where they sit, coil towers and then protect with WD40 or duck oil. Take the coils out and inspect the rubber insulation and clean and protect that area and retry..It wouldn't  cost too much to stick 2 new HT leads on first. If you can get a code reader it might have a P code for Crank sensor fault which will also cause backfire.

digitalscream

OK, looking at it the coil packs on the ZR are expected to last up to 120k miles. The car's done 116k, so it's not beyond the bounds of possibility that they've gone a tiny bit early (I have no service history to go by, so I'm going to assume that they've never been changed).

Seems prudent to replace the coils, HT leads and the spark plugs all in one go while I've got it opened up. That'll be a job for next weekend, then...

shaunyshaun1234

You can get a code reader off ebay for around a tenner that connects to your phone and use an app called Torque.

Might be useful to have and can read generic codes. It sometimes struggles to get specific manufacturer codes.

chillidoggy

Your mate will be hoping and praying it's the head gasket that's fucked.

Nitefly

I had a (brand-new!) Rover 620 that did similar things - started cold no problem, but as soon as it got warm started misfiring, loss of power, etc.  I took it to 3 different garages, none of them could sort it out.  

Eventually I left it with one of them overnight, so they could try it first thing in the morning.  It did the usual, started fine from cold, then started missing once warm.  

They disconnected the engine management system plug, and looked at the pins - one of them was bent over, so wasn't making proper contact in the socket, and as soon as it got warm it expanded, so was shorting against another pin.  They straightened the pin, plugged it back in - problem solved!

It's got to be worth a quick check, before you start spending...

mike_l

I'd change the coils and plugs whilst they're out.
However, I'd be checking everything is plugged in properly, I have seen vehicles which magically developed issues when some work has been done, and hey presto, there's a plug half out.

Axe_meister

So you are able to rev the engine with no load, i.e. the clutch depressed. It's only when there is load on the engine you get no power, i.e. it does not rev at all and the rev counter stays at idle?
So Throttle position and airflow seem be be reading ok (otherwise would not rev with no load).

Once the revs are up and you suddenly dump the clutch it keeps on revving.
Sounds like a timing issue at low revs which is happening when the engine is relativly warm.

When cold it starts perfectly and carries on whilst the engine is hot.You switch off and try again and you get the issue.
When idling after re-start does it try and hunt for a stable idle?

This is what I think is happening.

When cold the ECU is getting a good measurement of temperature and revolutions per min. The engine starts and is able to maintain idle (lets say 800 rpm).

When you switch off and the engine is warm and restart without load, enough fuel (governed by temperature/throttle position/revs) is able to get to the engine so start and idle, when you place load on the engine the revs may fall below a certain threshold at which point the ECU reduces either the amount of fuel or sparks incorrectly.
When you get the revs up and dump the clutch the ECU suddenly realises you have sufficiant reves increases the fuel flow and the engine stays above the rev threashold at which the problem happens until you switch off again.
It sounds like when the engine is warm the ECU is not getting a rev reading when revs fall below a certain threashold so reverts to default idle mode. I'd say a dodgy sensor or connections somewhere

tone1

When you do the plugs make sure you take one out at a time (less chance of dropping something into cylinders, especially the small coil mounting bolts) only use your hands on the plug socket extension to gently catch the plug threads and don't just wind the plug in with a ratchet from the start. You won't feel it cross threading with a ratchet..

robgilmo

Do yourself a favour, take it to a garage with some decent diagnostic equipment. EBay code readers are shite unless you spend a good few thousand quid , asking to diagnose car problems on a forum is a waste of time and I don't think you can afford to swap out parts randomly until you get the right one swapped out. An hours diagnostic wont cost you too much, driving your car broken while trying to self diagnose could cost you a lot more.

digitalscream

robgilmo said:

Do yourself a favour, take it to a garage with some decent diagnostic equipment. EBay code readers are shite unless you spend a good few thousand quid , asking to diagnose car problems on a forum is a waste of time and I don't think you can afford to swap out parts randomly until you get the right one swapped out. An hours diagnostic wont cost you too much, driving your car broken while trying to self diagnose could cost you a lot more.

Well, it seems prudent to replace the plugs, coil packs and HT leads anyway since I don't know the history of the car. That shouldn't cost more than £80 or so doing it myself, which is pretty easy.

I've got to say, @Axe_meister has described it pretty accurately, at which point I've got no chance of diagnosing it or fixing it properly. If the plugs etc don't sort it, then I suspect it'll be going to the local mechanic for a bit of TLC.