Mk2.5 cuts out but recovers at 4,000rpm

Technical Area Engine, Transmission, Exhaust etc
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Newcomer here. Please be gentle.

I’ve had my Mk2.5 for about six months. It’s a bit scruffy but generally works well, apart from one thing. Occasionally the engine cuts out altogether at higher revs, and recovers only when the revs drop to 4,000rpm. The point of cut-out varies but the point of recovery is always the same.

So, for example, if I am accelerating in 3rd gear, it may let me get to 6,000+rpm or it may suddenly die at, typically, 4,500. No power from the engine whatsoever. If I just let the speed decay, power comes back fully the instant the engine speed drops to 4,000rpm, and it will almost always then allow me to rev it as fast as I want. Alternatively, just declutching and throttling back for an instant is also enough to restore power.

It generally needs to be quite a lot above 4,000rpm, but not always. Cruising on the motorway at just a whisker over 4,000rpm the other day the engine would cut out, I’d slow down very slightly, revs would drop to 4,000rpm, power would com back and I’d accelerate slightly again. Happened maybe every 20 minutes or so.

Some days it doesn’t happen at all. Some days it happens frequently. It’s worse when the engine is cold and worse, I think, when the road is wet.

Has anyone any bright ideas? I’ve done a bit of Googling and have acquired spare coil packs (don’t think it’s that, because I lose all power, not half), cam position sensor and crank position sensor) and would welcome advice on the best order in which to try replacing them … if replacing them is worth doing at all.

Losing all power, even just for a few seconds, as you’re snicking past someone on an A-road is … character forming. I’d like to get it sorted.

 

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1.6L or 1.8L? My 1.8L at 4k and top gear is doing 80+ mph - I wouldn’t call that ‘cruising’ Rev limit is 7,200 rpm, so it’s not that - or you hit redline more than you think. Hitting redline means the engine virtually ceases to function, so rev counter drops rapidly, if you glance at it a second after, then it will show less revs than when you hit redline. Effect is like slamming on the brakes IMHO.

I tend to use the rev counter more than the road speed (MPH) as I know what the car should do in each gear, and I shift up or down accordingly. Only time I’ve reached or passed 4K in top is in France, where the motorway speed limit is higher. On the other hand, hit 4K or more in lower gears, when passing slower traffic.

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Sorry, should have said - it’s 1.8L.

It’s definitely not hitting the redline - nowhere near fast or noisy enough, and I have done tests (on days when the problem is recurring) when I can watch the rev counter at about 4,400 - 4,500 when cutout occurs and at 4,000 when power comes back. With the car in gear and on the level it takes around 4 seconds for the road speed and revs to decay. Cutout speeds seems to vary but power back is always at as near 4,000 as I can read on the rev counter.

At first I thought it was a third gear problem because it showed up in enthusiastic overtaking and was cured by changing up, but after a while I worked out that it’s purely revs related. Which gear I’m in is, as far as I know, immaterial, though I haven’t had the problem in 1st (or reverse!).

5

I had a similar problem on a previous Alfa which would loose power when revved, this also caused a set of error lights on the dash. After a lot of hunting it turned out to be a poor connection on one injector, vibration at particular engine speed caused the connector to disconnect which then caused the ECU to flag the fault and cut power, cured by cutting the old connector off and fitting a new one onto the harness.

Can you read the error codes and see if anything is stored which might help you track it down?

I’d be thinking something electrical being disturbed by the vibration at a particular frequency ( but might be totally wrong )

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Good point, check Amazon for reasonable priced code readers. The OBD socket is in the underdash fuse location. - Near you right knee.

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I gather that as a 2002 car I won;t have an OBD port, but a strange proprietary one which can only be “read” by plugging a bulb in and decoding the flashes. Have I been misinformed?

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Yes, you have been misinformed. Check where I said - the fusebox under the dash, the OBD socket is a longish one, on the right hand side, pretty obvious when you see it. My car was built October 2001, and I have one, so yours should also. The older different socket was under the bonnet, and needed an LED light to check, but while it is still in the same place on an NBFL 2001- 2005 you can’t read full diagnostics from it. I think it’s for ABS diagnostics offhand, but these are also readable from the main OBD port IIRC.

The code reader I bought (recommended by another member) from Amazon is a MS309 - SKU:QT110800200 and cost around £15 - well worth the investment. It should help you more than most of us can.

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Is’nt 4k rpm just when the Variable Valving kicks in? Just wondering if it’s a sensor.

Just a thought…and just a coincidence perhaps.

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I can’t answer that, maybe Robbie can. My impression, it kicks in the harder you stamp on the loud pedal, but who knows! Revs would no doubt come into it.

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Many thanks. I have now checked, and the OBD socket is there. I have a scanner, but I think it may be VAG-specific as it reports a link error, so I have ordered an MS309 as you suggest.

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Any conclusions yet? Do you read a fault code?

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Many thanks for asking - much appreciated. My nice new code reader had not arrived by the time I had to leave home for a work trip. I’ll be back over the weekend and, assuming the thing is there, I’ll be searching for codes then. Watch this space …

(My best guess, by the way, is that it’s a cam position sensor problem and the ECU is restricting things to 4,000rpm because below that the VVT hasn’t kicked in and therefore it can work out cam timing from the crankshaft position.)

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I was thinking the exact same thing, my mk2.5 vvti 1.8 kicks in at about 4k revs, that’s where it really starts to pull. It sounds related to the VVTi in some way.

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Thanks for the quick reply - it could be the VTCS solenoid valve. or the solenoid itself. My logic says if either are faulty, then the CAS (CPS) sensor would - as you suspect, regulate the output with a limit on revs.

P1512 would indicate a VTCS fault, with codes P1569 indicating low input and P1570 a high input. Sorry - no idea what this means, electrical or oil pressure? (I don’t see how you measure oil pressure?) I don’t have a Mk2.5 workshop manual, only a Mk2, so cannot say what it applies to.

That is, if my interpretation of the drawings is correct, VTCS I assume means Valve train control solenoid.
(Both are on the back end of the camshaft L/H side if you stand at the car front.)

According to my Mk 2 manual, the CAS is at the back of the camshaft, but when I mentioned this several years ago, I was told on the Mk2.5 it’s on the front. Not had a problem - thank God, so never had to search for it!
However, it seems logical if Mazda stuck the VVT valve and solenoid in the former Mk 2 position, then they would have to move the CAS to the other end of the camshaft. However, my Mk2.5 electrical manual shows the connector for the CAS in the centre loom at the rear of the engine - go figure.

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On mine it’s at the back right of the rocker cover - ie as near the driver as you can get. I bout a pattern replacement to try it, but although it fitted the 'ole, the original fits over a ridge on the rocker cover and the pattern one didn’t so I couldn’t try it.

Many thanks for the fault code advice … I may come back to you …

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Both might be correct.

The one on the end of the camshaft at the back of the cambox is the Camshaft Position Sensor (CMP or CAS).

If your crankshaft pulley has the extra disc (with ‘protrusions’ on it) fitted on the back, then that also has a Crankshaft Position Sensor (CKP or CAS2),

As far as I can tell, later MX5 engines should have both of these, and on two of my Vauxhalls it was always the crank sensor that failed, usually immediately after a service because a hamfisted mechanic had stressed its wiring when changing the air filter.

Crank position sensor should be between 0.5 and 1.5mm behind each of the protrusions of the disc on the back of the pulley. See page 01-40-13 in the 1999 Mellens manual, or Fig 42 etc in the 2005 Mellens manual

Cam position sensor and knock sensor are then covered in the subsequent pages.

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Quick note for Richard mainly, but applies to the OP as well. I’m giving part names from the Mazda Electrical wiring diagram book, not Mellens, so I assume Mazda are correct - at least for NE cars.
The crank angle sensor is labelled CPS (crank position sensor) and so is the camshaft sensor (CPS - camshaft position sensor) so when fault finding, and ordering spare, one needs to be careful. As for the position of the camshaft position sensor, as I said, the connector is located at the rear of the engine, so the Camshaft Position Sensor would hardly be at the front.

As for the crankshaft position sensor, The connector is shown at the top front of the engine loom, so while the actual sensor positions are not indicated at all (I don’t have the Mk2.5 workshop manual, only the Mk2 one) one has to go by location of the applicable connector - or wiring plug in other words.

Frankly, I don’t find Mellens wiring diagrams are easy to read, whereas the Mazda wiring manual is more specific. However, the Mk 2 workshop manual lacks detailed information, you need (apparently) dozens of supplementary sheets issued at intervals which only dealers get. I rarely use it, except if desperate!

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The MK1/Eunos 1989 - 1998 has a combined crank and cam sensor mounted at the rear of the engine - on the left for 1.6 and right for 1.8. There is confusion here as the part is known as a CAS cam angle sensor or occasionally crank angle sensor. It is a hall effect sensor that generates 2 pulses for cam and crank.  

MK2 and MK2.5 cars(1998 - 2005), 1.6 and 1.8 are all the same except 1.8 VVT - cam sensor is mounted on the front left of the cam cover. It’s on the top of the cam cover at the back left on the VVT.

There is a separate crank sensor mounted to the front of the block.

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Back home and the code reader is here. Plug it in … “one fault code found”. Try to read it … “no code is stored in the module”. Can’t clear it either.

Curses.

 

Edit: Tried replacement cam sensor anyway. Problem unchanged. More curses.