Car revs weird after timing belt change

[Pro car crasher]

Changed the timing belt on my mk1 MX5, seems to run okay at idle but when coming quickly off the throttle the revs drop, then bounce up again and drop back to idle. It makes a bit of a loud noise while it does this, sounds like it might be backfiring but I'm not sure. It seems like the noise is coming from the intake, I have just removed the old air box and put a new cone air filter in though so I would expect it to sound a bit different.

I recorded this video of the problem If anyone can let me know what might be going on that would be great

Cheers

Edited by Pro car crasher on Tuesday 10th July 18:56

[Mignon]

So what do you think the problem is?

[E-bmw]

My guess is the OP fked it up........ Just a guess like.....

[Pro car crasher]

Just taken it for a quick drive down the road, seems to be running alright, it's quite clear it's just backfiring when coming off the revs, I don't think the rev gauge is working properly though as it dropped down to zero at one point, and each time I saw the needle drop I din't hear any change in engine speed. I also noticed the gauge bounces a bit while accelerating as well as decrease in engine speed, so it seems the gauge is just not giving an accurate reading during a sudden change in rpm?

[E-bmw]

Sounds like it is just the gauge then..............

[Pro car crasher]

Might be worth noting that a 10 amp fuse that seems to be part of the dash has been blown on me about 4 times recently, which takes out the indicators (hazards still work though) rev gauge, oil gauge and temp gauge, so I wouldn’t be too surprised if there’s something wrong with the electrics going to the dash, anyone know what would be the best thing to check out in that area?

[Mignon]

I was just curious if the OP could tell the difference between what was obviously just the rev counter losing signal every now and then or the engine itself actually changing revs. Bad connection or bad earth somewhere in the electrics. Not a clue where to start looking. Intermittent electrics are probably the worst problem to try and find.

Also might be a hose not connected because idle is a bit flaky. Check around the inlet manifold and throttle body for air leaks.

[Penelope Stopit]

B METER 10 Gauges, Warning lights, Turn-signal lights, Cruise control
http://knigaproavto.ru/shemy/en/mazda/mx-5/388-198...

Pull out the 10 Amp fuse and check to see what doesn't work, the reverse lights could be on that fuse although they don't get a mention. Be sure to check everything on the car with fuse in and fuse out so you know everything that is on the fuse
Now get looking for a short on one of those circuits, reverse light switch wiring shorting out is a favourite if they are on that fuse

A short circuit for a split second may not blow the fuse but it will cause the rev counter to drop or become unstable

Play with the wiring (waggle test it) with ignition on, try to blow the fuse

[Pro car crasher]

It does seem more like the counter losing signal so probably a loose connection somewhere. I found one of the plugs under the dash was not connected properly which I've plugged back in but I've still got the same problem, the backlights on the gauge cluster are noticeably brighter than before though so there's that. I think it would be something to do with where the rev gauge connects to the engine, as it seems to happen under a sudden rpm change which could be causing the engine to shake and move a loose connection.

[Penelope Stopit]

Pro car crasher said:

It does seem more like the counter losing signal so probably a loose connection somewhere. I found one of the plugs under the dash was not connected properly which I've plugged back in but I've still got the same problem, the backlights on the gauge cluster are noticeably brighter than before though so there's that. I think it would be something to do with where the rev gauge connects to the engine, as it seems to happen under a sudden rpm change which could be causing the engine to shake and move a loose connection.

It could be that when the revs change the engine moves and any wire that's rubbing will short out and blow that fuse if on it

A split second short on the rev counter fuse is very similar to the loss of the rev counter signal

Then again there could be 2 faults, 1) a short circuit and 2) a rev counter or rev counter wiring fault

Edited by Penelope Stopit on Tuesday 10th July 21:29

[Pro car crasher]

Penelope Stopit said:

It could be that when the revs change the engine moves and any wire that's rubbing will short out and blow that fuse if on it

This seems like the most likely problem based on the way it moves, will have a better look in the morning when there's some light and not I'm pissing off the neighbours revving up the car.

[stevieturbo]

But you seem to be indicating everything was ok before you changed the belt...now this rev counter problem has only appeared thereafter ?

Or is there more to it ?

[Pro car crasher]

I did notice the rev counter bounce up and down a couple times before but it wasn't quite as extreme as it is now and it didn't happen too often so I didn't think much of it. It's now happening almost every time I rev the car whereas I'd only seen it happen a couple times before just after a gear change.

[buggalugs]

Is there a cam and a crank sensor on these, if so cam timing out could cause disagreement between the two sensors as to engine position and the ECU would have trouble reading RPM.

[stevieturbo]

buggalugs said:

Is there a cam and a crank sensor on these, if so cam timing out could cause disagreement between the two sensors as to engine position and the ECU would have trouble reading RPM.

Rev counter would generally be taken indirectly ) off the crank sensor. Cam sensor should not affect this.

But if there was a crank/cam trigger issue there should be an effect on how the engine runs, it appears this is not the case ?

[buggalugs]

stevieturbo said:

buggalugs said:

Is there a cam and a crank sensor on these, if so cam timing out could cause disagreement between the two sensors as to engine position and the ECU would have trouble reading RPM.

Rev counter would generally be taken indirectly ) off the crank sensor. Cam sensor should not affect this.

But if there was a crank/cam trigger issue there should be an effect on how the engine runs, it appears this is not the case ?

Seems like it misses to me when the rev counter drops? Was watching it on my phone so maybe I was mistaken. Got to be worth a double check of the timing and belt tension either way since he's just done the belt.

[Pro car crasher]

buggalugs said:

Seems like it misses to me when the rev counter drops? Was watching it on my phone so maybe I was mistaken. Got to be worth a double check of the timing and belt tension either way since he's just done the belt.

The engine doesn't sound like it's doing what the rev counter says it is, so I don't think the problem is completely to do with the timing belt, but it has just come up after doing the belt so I'm not sure. I'm not even going to be able to take a look for a while now as well due to work and other stuff. Always the way...

[stevieturbo]

But you said it did it before the belt...so it hasnt just come up after the belt ?

It's an ongoing problem

[Pro car crasher]

okay, so after trying to take a look at some of the wiring the problem seems to have gone away for now, I have unplugged and plugged back in a few connections so it might have been a loose connection I didn't notice? Haven't actually driven the car yet but I'll see how it goes once I get it back on the road.