[fortythirtyfive]

About 3-4 years ago I changed my plugs to NGK's and altered the gap (I can't accurately remember what to) I was advised to by a member on here at the time - I know many of you will probably now say otherwise.

On cold starts I have always had a stumble with the "start up roar" option that MHD offers, at first I presumed it was something that came as a trade off when the car was cold, however my friends 435 with the same tune and mods did not have this issue.

The stumbling start up goes away once the car is up to temperature, however there have been occasions where it stumbles so bad I thought the engine was going to stall. Just to be clear though this is only the first second of start up, not during idle.

I'm going to take a wild guess that running shorter gaps on the NGK's wasn't the best move? I run MHD's stage 2+ tune and have come to learn that the stock gaps, even stock plugs should be just fine?

Anyway I'm approaching 25k miles since changing them so I was going to service them. If I purchase the NGK's again (or even Bosch) but leave the gap stock; will it be likely to fix this?

This is probably the mildest example of what I am experiencing: https://www.youtube.com/shorts/DuETnskNGHM

IIRC that was filmed after idling for 2-3 minutes so it's not really highlighting the problem but gives you a slight idea.

[thejeremyman9]

No guarantee plugs will solve the problem but it sounds like you are due anyways and you might as well see before you try anything else. I run stock plugs stock gap on MHD stage 2+ E20 map. Also ran them on the 91 map, stage 1 and 2+. I also run startup roar and dont have any issues.

That video didnt even sound like startup roar was enabled but hard to say without seeing the RPM. Also startup roar shouldnt really affect a true cold start. On a cold start the RPM will raise and stay up for some time, whereas startup roar is just a quick blip of the throttle (at any engine temp) and it will quickly fall back down to idle.

[fortythirtyfive]

Originally Posted by thejeremyman9

No guarantee plugs will solve the problem but it sounds like you are due anyways and you might as well see before you try anything else. I run stock plugs stock gap on MHD stage 2+ E20 map. Also ran them on the 91 map, stage 1 and 2+. I also run startup roar and dont have any issues.

That video didnt even sound like startup roar was enabled but hard to say without seeing the RPM. Also startup roar shouldnt really affect a true cold start. On a cold start the RPM will raise and stay up for some time, whereas startup roar is just a quick blip of the throttle (at any engine temp) and it will quickly fall back down to idle.

I may even still have my old Bosch oem plugs somewhere, I'm pretty tempted to throw them back in and see if there's any difference. The only problem is I waited til 62k to change them and had ran the car for 5k miles tuned prior, so they're pretty well worn ha.

Yeah start up roar since I can remember has always been very underwhelming on my car. I have even had the odd person raise an eyebrow when it stumbles on a super cold engine.

[SuperLomi]

Did you disable the cold start raised idle when you flashed MHD?

[thejeremyman9]

Originally Posted by fortythirtyfive

I may even still have my old Bosch oem plugs somewhere, I'm pretty tempted to throw them back in and see if there's any difference. The only problem is I waited til 62k to change them and had ran the car for 5k miles tuned prior, so they're pretty well worn ha.

Yeah start up roar since I can remember has always been very underwhelming on my car. I have even had the odd person raise an eyebrow when it stumbles on a super cold engine.

I wouldnt put those old plugs back in.

I also have the same question as above. If you disabled cold start but enabled startup roar that could be part of the issue.

[fortythirtyfive]

Originally Posted by SuperLomi

Did you disable the cold start raised idle when you flashed MHD?

Yeah, I'm running downpipes so it was the first thing I did, I used to set off for work at 10/11pm and the neighbours despised it luckily I now work normal hours.

I'll order a set of Bosch plugs, I can get 6 for under £50 so worth a shot.

[thejeremyman9]

Originally Posted by fortythirtyfive

Yeah, I'm running downpipes so it was the first thing I did, I used to set off for work at 10/11pm and the neighbours despised it luckily I now work normal hours.

I'll order a set of Bosch plugs, I can get 6 for under £50 so worth a shot.

If your issue is primarily on cold start then disabling the cold start is almost certainly the main cause. This is commonly reported. Startup roar isnt a solution, that's more of a hot-start add on than a cold start fix

[fortythirtyfive]

Originally Posted by thejeremyman9

If your issue is primarily on cold start then disabling the cold start is almost certainly the main cause. This is commonly reported. Startup roar isnt a solution, that's more of a hot-start add on than a cold start fix

Ah fairs okay.. so cold start delete could induce fluttery/stumbling cold start ups?

I suppose I could re-enable it? It just felt kinda pointless after I remove the cat.

[thejeremyman9]

Originally Posted by fortythirtyfive

Ah fairs okay.. so cold start delete could induce fluttery/stumbling cold start ups?

I suppose I could re-enable it? It just felt kinda pointless after I remove the cat.

Yeah. There's threads on this. Its primary function may be heating the cat for emission control but it also helps the car stabilize the idle when everything is cold. If you re-enable it and the car still behaves funny on cold start there may be another problem.

[arekuf3x]

Check your injectors, make sure they don’t leak. I had start up stumble, turned out to be one injector leaking.

[fortythirtyfive]

Originally Posted by arekuf3x

Check your injectors, make sure they don’t leak. I had start up stumble, turned out to be one injector leaking.

I guess when I do my plugs on Saturday I can check them, am I right in thinking I can get a rough indication by smelling the tip of the plug for leaked fuel? I remember being told that by a real old mechanic many years ago

[fortythirtyfive]

https://www.youtube.com/shorts/aMNlUlIlnoI

First take: start up roar enabled, cold start disabled
Second take: start up roar disabled, cold start disabled
Third take: cold start enabled

So the general feeling is it's just the cold start feature causing it. I'll leave cold start on for now, I forgot how obnoxious it is, granted I'm running a downpipe but that's still valves closed and running through a resonator and backbox haha.

[yupetc]

I don't have the stumbling issues you have, for reference point below are my parameters:
- Catless DP
- OEM spark plugs, stock gap.
- MHD stage 2+ 93 Oct.
- Startup roar enabled
- Cold start disabled.


Also, once your car is up to op temp, see if you're able to run the oil level check through iDrive. I often use that to help me gauge if there's a throttle control related issue ghosting around. During oil level read, engine will be controlled to rev at 1k rpms, and if there's something wrong, it'll not complete the read and you know there's an issue somewhere.

[Kopone]

I used to have cold start disabled and start roar enabled without issues on MHD stg1.

Now with stg2+ i have everything enabled.

[arekuf3x]

Originally Posted by fortythirtyfive

I guess when I do my plugs on Saturday I can check them, am I right in thinking I can get a rough indication by smelling the tip of the plug for leaked fuel? I remember being told that by a real old mechanic many years ago

Yes it would definitely smell like fuel, also another way to check rail pressure using mhd monitor. After 30min drive when car is fully warmed up, turn it off and see if rail pressure drops, it should slowly rise.

[fortythirtyfive]

Originally Posted by yupetc

I don't have the stumbling issues you have, for reference point below are my parameters:
- Catless DP
- OEM spark plugs, stock gap.
- MHD stage 2+ 93 Oct.
- Startup roar enabled
- Cold start disabled.


Also, once your car is up to op temp, see if you're able to run the oil level check through iDrive. I often use that to help me gauge if there's a throttle control related issue ghosting around. During oil level read, engine will be controlled to rev at 1k rpms, and if there's something wrong, it'll not complete the read and you know there's an issue somewhere.

Praying for spark plugs then, I have OEM Bosch ones on the way.. this time I'll leave the gap alone ha.

I just did an oil change the other week and ran the test which completed fine.

Originally Posted by arekuf3x

Yes it would definitely smell like fuel, also another way to check rail pressure using mhd monitor. After 30min drive when car is fully warmed up, turn it off and see if rail pressure drops, it should slowly rise.

Cool, I'll make sure to check that.

The only positive to enabling cold starts is everything felt primed and ready to go earlier, almost like the car was 5 minutes more awake than usual.

Man it's been an expensive month.. pads, rotors, had some extra exhaust fab work done at £500, now plugs.. here's to hoping the injectors stay happy.

[arekuf3x]

I’m running oem Bosch plugs and oem gap on pure500 turbos with custom mhd e30 tune, intercooler and high flow cat, b58 hpfp, never had misfires other than one injector. She feels very strong. Cold start disabled with start up roar and no issues.

[fortythirtyfive]

Plugs arrived a little earlier than expected in the morning, changed them out on my lunch break.. 16 minutes in total from start to finish tided up, new record? ha.





The NGK's were gapped to .22, the Bosch ones are .30 all round.

Sadly the stumbly start up is still a thing when cold start up is turned off, however I'm just going to presume this is a characteristic to live with when disabling cold starts. I'll try get a log later however I had a quick 3 minute blast up and down the road and everything feels buttery smooth.

[yupetc]

Originally Posted by fortythirtyfive

Plugs arrived a little earlier than expected in the morning, changed them out on my lunch break.. 16 minutes in total from start to finish tided up, new record? ha.





The NGK's were gapped to .22, the Bosch ones are .30 all round.

Sadly the stumbly start up is still a thing when cold start up is turned off, however I'm just going to presume this is a characteristic to live with when disabling cold starts. I'll try get a log later however I had a quick 3 minute blast up and down the road and everything feels buttery smooth.

I'm quite sure you've already known to do this, but reset all the adaptations through the MHD app and see if that changes anything. After this point, I'd try to pull codes daily and see if any ghost codes may start creeping up. Sometimes a smoke test helps, too if there's an air leak. There is no other clue to where we need to look, but some of the common and less costly things, plus taletell from driving the car and be keen on the differences may help.

[fortythirtyfive]

Originally Posted by arekuf3x

Yes it would definitely smell like fuel, also another way to check rail pressure using mhd monitor. After 30min drive when car is fully warmed up, turn it off and see if rail pressure drops, it should slowly rise.

Originally Posted by yupetc

I'm quite sure you've already known to do this, but reset all the adaptations through the MHD app and see if that changes anything. After this point, I'd try to pull codes daily and see if any ghost codes may start creeping up. Sometimes a smoke test helps, too if there's an air leak. There is no other clue to where we need to look, but some of the common and less costly things, plus taletell from driving the car and be keen on the differences may help.

I'll give those a try tomorrow, I was always told to leave adaptations alone so I never dared haha. I think they did a smoke test on my last dyno day just to be sure. With all of that said, and this may be placebo.. but the car feels way smoother and a little more alive on these Bosch plugs.

Maybe that's because the others were old and used, but they were also gapped pretty small which they probably didn't need to be anyway I'll try get a clean log tomorrow - I tried doing one earlier but they've just resurfaced my local stretch of roads around here and the loose gravel was nuts (here in the UK they rely on cars to drive over the stones and push them into the tarmac) I did hit 18.4 psi on boost though, normally it maxes at 16.

[thejeremyman9]

Its very possible the OEM plugs are giving you better combustion with a bigger gap. Also FYI you arent supposed to use a feeler gauge on these type of plugs, you're supposed to use the wire tool to avoid damaging the tip.

[fortythirtyfive]

Originally Posted by thejeremyman9

Its very possible the OEM plugs are giving you better combustion with a bigger gap. Also FYI you arent supposed to use a feeler gauge on these type of plugs, you're supposed to use the wire tool to avoid damaging the tip.

Yeah I remember reading this somewhere, I ended up eyeballing .32 and .30 on the Bosch ones as I had a rough idea it would be around that.