M20B20 sputtering when warm

[l0gistaja]

Howdy. So I've been having this problem with my E30. The car runs well when it's cold. Revs up nice, everything is good. But when the engine reaches ~90*C I start to get sputters. When I slowly press the gas pedal, it works fine but if I were to floor it a little bit quicker it would sputter. I recently cut most of the floor out as well and as the engine warms up I've been sensing a weird smell coming from the passenger side, right around where the catalytic converter is.
At first I thought this problem was caused by a broken fuel line but now that I've fixed it, the issue hasn't gone away.
I suspect it might be the battery which is quite old and needs to be charged like every two weeks or so. It might be the cat too, because it kinda looks bad from the outside. From what it looks like, I would say that someone has kinda built a cat from two broken cats or something similar.
Does anyone have any idea about what could cause this?

[peterman]

I would start with a new distributor cap, rotor and plug wires

Does it sputter or does it seem like its not firing on all cylinders when you floor it?

[e30davie]

Based on the description there are alot of things i would do before blinding spending ~$300 on a dizzy cap rotor and and wires.

Here is how i would attack it.

Absolutely first check for vacuum leaks. Vacuum leaks are responsible for 95% of issues with M20s. Although maybe not your issue, its always a good place to start. quick check of all clamps and hoses will take you 10mins.

Secondly, with the car running put a multi meter on the battery, should be over 12.5-13V when running and 12ish when off. This will confirm if your alternator is working, if it is not over 12.5-13V you have alternator issues. Fix alternator before proceeding any further. If your battery is actually stuffed replace before proceeding any further.

You say it runs good cold, but goes to shit when hot. First thing that points me to is the ECU temperature sensor. Remove it, clean it up and check the resistance based on the data given in the bently manual.

Next up CHECK the dizzy cap, rotor and wires. Inspect the cap and rotor for wear and corrosion, If they are a bit corroded get into them with some sand paper and clean them up, if this helps then you found your problem. If it doesn't keep searching but put new dizzy cap and rotor on the list of things to buy. Find the resistance values for the wires in the bently manual, check with multi meter. also check the coil while you are there.

Also remove spark plugs, clean up with a wire brush and re-gap. If they look a bit sad get some new ones, but a clean up with a wire brush and re-gap can give enough life back into them for diagnosis purposes.

Your exhaust description is difficult to understand but if you have a blocked exhaust then that wont be helping.

The above should get you quite a way towards correct diagnosis of your issue.

[l0gistaja]

Thanks for the fast responses. The issue seems to have solved itself.
I checked the voltage on the battery with the engine running. The reading was around 13.8V so the alternator is all good.
Did a quick brake cleaner test for vacuum leaks as well, didn't find any.
And just as I was about to make a video to explain myself bettter, the problem just went away on its own. Now the engine is running as smooth as ever.
My guess is that it might be the corroded dizzy cap and rotor (haven't checked em yet). Now that the engine runs good, I don't see as much smoke coming from the exhaust as there was before too.