MK2 8v digi(pissmeoff) - HELP - revs drop regularly when held constant throttle

[courtz1976-2dr]

Hi there everyone

Im hoping someone can point me in the right direction with a new problem

when I hold the throttle steady at any revs the revs drop regularly but always back up to relative to throttle

ie hold @ 2000 smooth - drop - smooth up to 2000 - drop
hold @ 3000 smooth - drop - smooth up to 3000 - drop etc etc

it also has another weird issue when accelerating hard after 4500rpm it hesitates and feels like misfire, accelerate a little softer and it revs fine all the way to redline.

i have new MAF, dizzy, cap, leads and plugs seem ok coil feels a bit hot to the touch

grrr

[h100vw]

Hi there everyone

Im hoping someone can point me in the right direction with a new problem

when I hold the throttle steady at any revs the revs drop regularly but always back up to relative to throttle

ie hold @ 2000 smooth - drop - smooth up to 2000 - drop
hold @ 3000 smooth - drop - smooth up to 3000 - drop etc etc

it also has another weird issue when accelerating hard after 4500rpm it hesitates and feels like misfire, accelerate a little softer and it revs fine all the way to redline.

i have new MAF, dizzy, cap, leads and plugs seem ok coil feels a bit hot to the touch

grrr

I wouldn't go with plugs seem ok, try a new set for the cost of them. If nil fix, take them out and use them in the future. The coil will get hot under the bonnet and due to the power going through it.

The miss, does that happen with your foot on the boards,. ie in open loop or is it not quite at WOT, closed loop?

Fuel filter?

Throttle switches?

Knock sensor and wiring

Gavin

[courtz1976-2dr]

Hey gavin thanks for input

The miss happens after 4500 rpm when throttle opens up the second butterfly all the way to WOT... (in open and closed) correct me if I am wrong but from fully closed throttle position to fully open throttle position - connection should read closed - open (mid ranges) to closed (full throttle) with multimeter across terminals

The problem with the revs dropping only surfaced after I managed to fix the wiring to throttle sensors but miss/hesitation has always been.

the revs drop in a very orderly pattern up 3 sec down 1 up 3 sec down ... like no spark/fuel at all during "down" section (idles fine)

I thought my miss problem was the throttle sensors not "fuel enriching" at flat to the boards.

the knock sensor is sad looking but how can I bypass/test ???

I rebuilt the entire mechanical parts of the engine and know that it is being held back by these annoying issues.

[h100vw]

Whack the bolt in the knock sensor with something at idle. the revs will change if it's working. Disconnect it at the plug and make sure none of the wires inside are shorted together or shorted to the screen around the cable.

Not sure which way the switches go without a diagram in front of me. As long as they change state, open to close or vice versa, I'd be pretty confident they are working.

The hunting, if at idle would definitely be the the ISV but I don't recall seeing one do it mid range. Have really good look for air leaks. Spray some WD40 or similar over the pipe work and see if it revs up differently at idle.

Gavin

Hey gavin thanks for input

The miss happens after 4500 rpm when throttle opens up the second butterfly all the way to WOT... (in open and closed) correct me if I am wrong but from fully closed throttle position to fully open throttle position - connection should read closed - open (mid ranges) to closed (full throttle) with multimeter across terminals

The problem with the revs dropping only surfaced after I managed to fix the wiring to throttle sensors but miss/hesitation has always been.

the revs drop in a very orderly pattern up 3 sec down 1 up 3 sec down ... like no spark/fuel at all during "down" section

I thought my miss problem was the throttle sensors not "fuel enriching" at flat to the boards.

the knock sensor is sad looking but how can I bypass/test ???

I rebuilt the entire mechanical parts of the engine and know that it is being held back by these annoying issues.

[courtz1976-2dr]

Whack the bolt in the knock sensor with something at idle. the revs will change if it's working. Disconnect it at the plug and make sure none of the wires inside are shorted together or shorted to the screen around the cable.

yeah its ****ty - but can I not disconnect it or bypass somehow just for testing - will do a bit of googling for this or if you know (it definitely needs replacing)

Not sure which way the switches go without a diagram in front of me. As long as they change state, open to close or vice versa, I'd be pretty confident they are working.

Yep all good here

The hunting, if at idle would definitely be the the ISV but I don't recall seeing one do it mid range. Have really good look for air leaks. Spray some WD40 or similar over the pipe work and see if it revs up differently at idle.

agreed

I just went out there then and held at constant revs and listened to the injectors, I can hear them cut out and back in again only after getting past about 2000rpm all the way up and only when holding steady throttle position - now knock sensor doesnt cut fuel right??? just retards ignition timing???

I really like this little car and it has been so faithful -

Courtney

[h100vw]

Injectors cut out as the engine decelerates, then come in again before about 1500 revs to stop the engine stalling.

Knock sensor just affects timing, correct.

The ECU will recognise if you disconnect the knock sensor. How much this affects an ECU from the 90s I have no idea. Essentially, you need to have one that works connected.

yeah its ****ty - but can I not disconnect it or bypass somehow just for testing - will do a bit of googling for this or if you know (it definitely needs replacing)



Yep all good here


agreed

I just went out there then and held at constant revs and listened to the injectors, I can hear them cut out and back in again only after getting past about 2000rpm all the way up and only when holding steady throttle position - now knock sensor doesnt cut fuel right??? just retards ignition timing???

I really like this little car and it has been so faithful -

Courtney

[courtz1976-2dr]

Ok another question,

when the throttle is held open a little so that idle switch clicks, if i depress the top thrttle switch what effect should this have (if any) on revs etc. i have done this test and (engine hot) and the engine revs drop smoothly and rises again when not pressed - this is contrary to what i thought for a full open position I thought it gave more fuel???

Any more ideas on the original hesitation would be great - ill get some new plugs and try but I dont think these have done much more than 5000k (new ones at rebuild)

[h100vw]

Ok another question,

when the throttle is held open a little so that idle switch clicks, if i depress the top thrttle switch what effect should this have (if any) on revs etc. i have done this test and (engine hot) and the engine revs drop smoothly and rises again when not pressed - this is contrary to what i thought for a full open position I thought it gave more fuel???

Any more ideas on the original hesitation would be great - ill get some new plugs and try but I dont think these have done much more than 5000k (new ones at rebuild)

You are devising situations that aren't going to happen in reality. WOT with no actual butterfly movement, plus no large movement of the airflow meter.

The fuel is injected based on airflow/load, not the switch alone. The switch puts the ECU into open loop, ie it stops trying to reference the lambda probe for mixture regulation. It just runs off a more basic map.

[benough]

May be a bit off the path, but our RV 8V used to hunt at idle. The idle cutoff swtich plug, one of the wires had fallen out.

Plugged it back in and idled like a dream ever since.

[golfmeup]

I had a crack in one of the vacuum pipe connectors that was causing some odd rev- idle issues.
Have you checked all of these?
Only takes a few minutes or so to see.

Hope you get it's sorted soon.