Cut out

[SuperRedMR2]

Hi Paul,

The timing has been checked and checked again by a chap with extensive knowledge like you, so it's not that.

Where could the air leak be coming from?

I'll do the other checks when my g'box has been replaced at the weekend.

Cheers

[PW@Woodsport]

When you say timing, I presume you mean both cam and ignition?

Air leak could be from anywhere on the intake side of the head to the throttle body.

Check the TPS is set correctly.

[SuperRedMR2]

yes mate both.

[jimi]

When the engine up to normal temperature, it idles around 1000rpm, I took the inlet hose off and covered the hole and the revs dropped considerably. It was stuttering a bit.

So?

To me if it's idling at 1000rpm warm, if you block then off the inlet port for the AAV and the idle drops considerable, then the AAV is not fully closed and is is passing air ... or am I missing something ?
AFAIK that port only goes to the AAV which bypasses the throttle butterfly when its (the AAV that is) open, which is same thing as an air leak to my way of thinking ??

[PW@Woodsport]

The Waxstat will always flow some air past it even when fully warm and working as it should be. Of a typical 900rpm idle I'd say a good 600rpm of that is air going past the waxstat valve, the additional 300rpm being added by the air bleed screw. A Mk1 will cut out or come very close to stall with the bypass hole covered, so I honestly don't think this is an issue.

It sounds to me like basic settings are not right.

[tottacrolla]

If you are going to the trouble and expense of fitting performance cams then they should be timed to make power, not to help the engine idle better.
Catcams typically have quite a lot of overlap and this will effect the idle vacuum. Expect the engine to run rich at idle because of this and upping the idle speed will make little or no difference.

[PW@Woodsport]

If I understand correctly, it's not that it doesn't have a steady idle, more that it's dropping out on the overrun? Coming up to junctions?

[SuperRedMR2]

Hi tottacrolla,

I originally thought the cams were at fault for the idle issues I am experiencing, come to the conclusion they have nothing to do with it.

I have experiencing the engine cutting / dropping rpm coming up to junctions etc when the engine is hot.

[jimi]

The Waxstat will always flow some air past it even when fully warm and working as it should be. Of a typical 900rpm idle I'd say a good 600rpm of that is air going past the waxstat valve, the additional 300rpm being added by the air bleed screw. A Mk1 will cut out or come very close to stall with the bypass hole covered, so I honestly don't think this is an issue.

Except if you have certain persons kit fitted in which case the waxstat inlet port is blocked off and all the idle air is through the air bleed
My thinking was along the lines of if the waxstat is passing air (more than normal) then the idle may have been to high and has been reduced by use of the airbleed screw, if the waxstat then decides to shut properly (after/during a longer/harder run than usual) the idle speed would then be too low, resulting in the engine cutting out on over run ?? obviously just a theory on my part, could be total rubbish

[tottacrolla]

Could be that the TPS is not switching the ecu into 'idle mode' via the idle switch.

[PW@Woodsport]

Could be that the TPS is not switching the ecu into 'idle mode' via the idle switch.

Yep, already suggested that.

I get you now Jimi, yes with my manual choke all of the bypass air is coming via the bleed, i suppose if the stat is closing more than expected randomly it would definitely stall out..... but i find it hard to believe that is only happening after being on the overrun coming to junctions, although there is maximum vacuum being pulled through the bypass on overrun.... hmmm.

[tottacrolla]

Could be that the TPS is not switching the ecu into 'idle mode' via the idle switch.

and a code 11 would confirm it.

[jimi]

... but i find it hard to believe that is only happening after being on the overrun coming to junctions, although there is maximum vacuum being pulled through the bypass on overrun.... hmmm.

yes, that bit is the fly in my ointment! would be a hell of a coincidence !

[SuperRedMR2]

It does only happen when coming to halt approaching junctions etc and sometimes the revs drop down too low if i'm changing from 2nd to 1st.

Gearbox swapped now, so will pull off all my vacuum lines, check them all out and make sure everything is as it should be, including the TPS.

[SuperRedMR2]

I've seen an interesting video on youtube and spoken to my ageing mechanic who also said he suffered from exactly the same issue on his mint mk1b.

I've taken the throttle body apart and cleaned all the xxxx out of it.

Will see how it runs after i've done the nut up tight on the starter motor connection!

[SuperRedMR2]

Since cleaning out the AAV or whatever is that is bolted to the throttle body it runs a lot better. No cutting out at lights, no dipping of RPM, nothing.

[jimi]

Yes, AAV is bolted to the bottom of the throttle body, sounds like cleaning it out has cured your problem.

[SuperRedMR2]

It didn't work - it came back in vengeance yesterday - going to replace all my vacuum lines. I had a slow bouncing idle yesterday each time before it cut out, so it must be a leak.

Cleaning AAV did cure it originally though.

[PW@Woodsport]

Like I said the waxstat valve has nothing to do with maintaining a hot idle, I knew that wasn't your issue. Test this if you like, block off the waxstat totally with a plastic tyre valve cap and raise your idle bleed screw to 900rpm, bypass air is bypass air no matter how it gets past the throttle..... I bet you £100 your issue is still there with the waxstat completely disabled.

Its purpose is purely to add "extra" bypass air when the engine is cold, once hot the ECU is not expecting to see any additional plenum vacuum drop.

As before, check the TPS is on its idle switch and working correctly, you can put a timing light on the crankshaft and at idle note the timing is 16* (without T and E1 bridged), then open the throttle slightly and it should jump up, if your base timing is 30* or something at idle then the TPS is not set on its idle switch.

[SuperRedMR2]

Matters have not been helped as I've found the TPS plug is not actually plugged in...I didn't connect it again once I re-installed the throttle body.

I'm sure that doesn't help...