Fixed - M88 Rough Idle, Noise, Vibration till warm.

[passta]

85' M635.... Having an issue with a rough idle and lots of noise from the engine at startup, as in once it starts when cold, it will idle/sputter/miss at much lower than 850 it's set at and makes a lot of noise from the front of the engine, until it warms up, then it runs/idles fine, no noise. I typically will start it and feather the gas to get it stable after a few seconds, but the noise/shaking is a lot. At first I thought it was the ambient temp here in St. Louis as it was still cold in the morning, 30-40 degrees in my garage, but as its warmed up to spring its still occurring. M88 engine, Timing chain and guides were done along with a ton of other stuff, belts, hoses, oil pump rebuilt, water pump, new exhaust, headers, within the past 1000 miles or so, by my friend @brickwhite, who I bought the car off of this spring. Oil level is fine, no smoking or anything, no vacuum leaks, nothing is obviously loose near, above, or below where I believe the noise is coming from, but it does sound like the chain area, but then goes away once it warms up. I can see oil moving around if I open the Oil Cap while its running, so everything looks like it's doing what it should and after its warm and idles at 850 on its own, no issues at all, runs and drives great. Taking it in to my mechanic next week but curious what anyone thinks this could be.

Link to video https://youtu.be/X9GrwthJWWA

[Ralph in Socal]

I’m not expert on the M motors but isn’t the front crank hub a known failure point? I would check that asap as worsening can then damage the crank as well. Good luck

Ralph

[passta]

I’m not expert on the M motors but isn’t the front crank hub a known failure point? I would check that asap as worsening can then damage the crank as well. Good luck

Ralph

That's been replaced same time as the chain and everything else as well.

[bpoliakoff]

Could the noise come from a worn out chain tensioner??

[passta]

I thought the same because it sounds like it but it is new as well. Not to say it couldn't be bad or something though...

[Lbs6]

What about the oil pump chain tension, it needs to be shimmed properly.

[wattsmonkey]

Rough idle only sorted on my M with properly balancing tbs with manometer.

That noise sounds to me like something loose after your rebuild - hopefully nothing internal, but definitely worth checking pulley to crank hub bolts, crank hub main nut, tensioner mounting nut etc. I've found fastenings that I've marked with paint that have loosened after a few thousand miles - drive shaft (or whatever you call the shafts that take drive from the diff to the rear wheels) allen-headed bolts being my most serious culprit, probably because there was oil in the threads.

Hope you get it sorted soon so you can enjoy that banshee wail as you wind it up! A glorious noise.

All the best,

Rob

[hffvdbl]

I wonder if the cold start injector is bad. Mine does this when its cold and I know my cold start injector isnt really doing anything. But when mine warms up its fine as well. I'm fairly certain mine has to do with the Johnson box that was installed when it was federalized in '85. I unhooked mine last weekend thinking that would help but I need to go back in and remove the wiring and repair the connection to the coolant temp sensor that the motronic uses for cold start and fuel enrichment. From what I've read the Johnson box intercepts this signal and uses an O2 sensor and potentiometers to work like a piggyback O2 sensor. Sorry for the long reply but you might be experiencing the same issue.

On cold start, disconnect the Cold Start injector and see if that makes any difference. It's the blue Bosch connector on top of the Throttle bodies.

[passta]

I've reached the limits of what I can do in my limited time and garage space and am going to have it checked out this week by the shop I use. I'll post back what the outcome is hopefully this week.

[passta]

'Ralph in Socal' and 'Wattsmonkey' you were both right. Issue was loose, actually missing, bolts on the crank hub. 4 of the 8 were no longer there and had worked there way out. Had those put back in with thread locker & tightened to spec, and then the remaining 4 removed, checked and thread lockered those, and tightened. The hub itself which was a new hardened one from BenGerman on mye28.com was fine and no other damage done luckily.

[wattsmonkey]

Glad you got that sorted before expensive damage!