What Drives The Tachometer?

[24seven 104]

I've suddenly started getting occasional complete loss of power accompanied by the tachometer dropping several thousand RPM before the power and gauge suddenly start behaving again. the car stutters violently like it's kangarooing, but it only jumps once. It's bizarre, the power loss only lasts a fraction of a second, miliseconds maybe and has no relation to engine/road speed, gear or throttle input. Any ideas what it could be?

[stuart1298 6]

do any lights come on, or go off??

 

battery lead, connector, earth strap?

[24seven 104]

All the lights & other instruments carry on as usual, literally just the tacho. Where are the earth straps on these things? Wiring's not my strong point and the only one I've noticed is on top of the gear box.

[Paul_13 212 1 Cars]

Fuel pump pressure dropping?

[24seven 104]

The fuel pump stays active as I can hear it. It's fairly new I think too. Literally just the power drops and tacho dives a few thousand rpm despite the engine not slowing down.

[welshpug 1,622]

surely if the power drops the engine speed will drop?

 

sounds like you are losing spark, which would go hand in hand with the rev counter dropping, it gets signal down wire 112 from the coil.

 

on ML4.1 the live for the coil is fed from a relay, on M1.3 its just an ignition switched live.

 

if you still get fuel injector earth signal to the ecu firing then TDC is fine.

 

is the fuel pump wired as normally would in an Mi16 loom?

[24seven 104]

The engine speed drops as long as the power drops yes, but it's only a split second loss in power and it definitely doesn't drop the few thousand RPM that the gauge is telling me. The pump is wired as normal, but I'm not sure if I have the relay for the coil despite mine being 4.1 - I had some issues tracing the switched live when I first converted the car and didn't come across one then. Would this relay be in the black fusebox found in the engine bay on the Mi16 loom? I've a set of spares I might try just in case.

[Cloverleaf 1]

The engine won't drop as much as the gauge as the wheels are still turning the engine over.

 

Basically your engine isn't firing for a second or so and your rev counter reacts quick enough to drop right off.

[24seven 104]

It's not like a normal misfire where the tacho still reads the engine speed as it actually is, that's another problem . Just been for a spin and I've got a video of it I'll try and upload now.

[Simes 248 2 Cars]

Do your headlamps (when on) go off also?

[Veero 1]

Sounds similar to what mine was doing even after replacing most of the ignition system, but mine would cut out and not restart straight away. Coil, leads, plugs, arm, rotor. Hadn't changed the amp, which I have now done with a known good one and taken it for 3-4 good rags and no repeat symptoms.

 

Rev counter dropping does sound like either coil not firing or amp not working.

[Tom Fenton 1,525]

Rev counter dropping is due to it not getting a signal, I'm suprised no one has suggested the crank sensor, seeing as this gives the ECU the info which governs when the coil will fire.

[24seven 104]

All those bits you named I have new as well Veero. Coil, dizzy, rotor arm, amp, plugs and leads, but since changing all those this issue has surfaced. I'd suspected the crank sensor may be partly to blame for the constant misfire I've been having. I've got another one handy so I'll give it a try and see if it makes any difference.

[C_W 3 1 Cars]

It's most defninitely ignition related. Mine suffered this when the ht lead from the coil to the dist cap was arcing/poor connection. It won't be fuel related as it's so sudden, or an individual ignition circuit/htlead/spark plug etc as it's all or nothing power drop.

 

I would also check the two relays on the Mi16 look which normally sit on the drivers inner wing, I forget which one, but this can also cause this problem.

[Veero 1]

Ah sorry forgot to mention I put a brand new Pug crank sensor on. Mine still died until I changed the ignition amp for a known working one.

[24seven 104]

hmm. I'll change teh crank sensor anyway as I have a known good one sat next to me. I've 3 amps to choose from, 2 bosch Mi16 amps which I've no 100% guarantee are fine and the one that was fitted to my 8v engine which was fine as an 8v and fine with my last Mi16 engine. That said neither of them seem to make any difference to how it runs, so I guess it rules that out.

 

I've a couple of coil-dizzy leads I could try, though I've not long since put a full set of known working very low mileage leads on which made little difference, and a new coil which only had a few hundred miles on it, again making no real difference. In fact the only thing that has made a difference is changing the exhaust manifold/downpipe gasket after the old bolts fell out! I guess the loss of back pressure just exacerbated things. I found out this week that our uni has an ANSU injector cleaning machine, so I'll try and put them through that next week if they let me.

[24seven 104]

Ok, I've got a vid of what the tacho's doing. When it happens the power is cut instantly, then comes straight back in, it's really quite violent, which the video doesn't show. You can see that the momentum of the car keeps the speed constant so the change in tacho reading isn't reciprocated by the engine. It's a LOT worse when the engine/engine bay are warm as well, from cold it doesn't really happen, but once it starts it gets more and more frequent.

 

[mang 3]

wheres your fueling relay? are you sure its not getting to hot and cutting out. Have you tried putting all the old parts back on the car and see if it goes away?

[24seven 104]

Fuel relay is in the engine bay with the ECU. Don't think it's that as it's too instant to be a drop in pressure. I'll try one of the old coils/amps/coil-dizzy leads tomorrow morning see if that changes anything.

[welshpug 1,622]

you should have two relays

[PVFCpug 6]

I had a similar issue with my MI last year. Had the 8V dizzy set-up with twin 45's. You could be accelerating hard and sometimes it felt like a kick in the back it was so sharp a drop in power. I went down the route of changing everything we have all mentioned to no avail. I changed Dizzycap, rotor arm, HT Leads, Small dizzy to amp lead, Amp itself, Tachy relay, Coil, and all the earths. Added additional earths just in case and coud not resolve it.

 

However on the way back from on of our meets the car died completely and Blue Haddock managed to get me home by jump leads putting a little charge into my battery and crawling home. Turned out the alternator had died a death. Got a new one installed and guess what no more sudden loss of power. Don't ask me how but it has 100% disapeared.

 

Not saying that is defiantely the problem, but that was my exerience.

[24seven 104]

Tried the old coil/amp/lead earlier and it didn't make much difference. It only happens when the engine bay is warm though, rather than the engine itself. I can get the engine up to temperature whilst running and it'll behave itself but if I stop and heat dissipates around the engine bay, next time I get in it the problem is horrendous! At least that's what I think is going on.

Edited February 1, 2010 by 24seven

[C_W 3 1 Cars]

Fuel relay is in the engine bay with the ECU. Don't think it's that as it's too instant to be a drop in pressure. I'll try one of the old coils/amps/coil-dizzy leads tomorrow morning see if that changes anything.

 

Try and change both the relays (as I said I forget which is which, but one is ignition, one fuel pump), it will likely be the former.

 

That or rotor arm is my bet.

[24seven 104]

I'll see if I can get hold of some relays for it tomorrow. The rotor arm is new, along with the dizzy cap.

[C_W 3 1 Cars]

I see, it's perhaps less likely to be the rotor arm as that would probably give a lighter general mis-fire than a full-on ignition cut (like the relay would).