I did have a good look around the forum and have read a bunch of threads about these not uncommon issues with the C3.2 so I
think I have the beginnings of a handle on how to try and diagnose the issues. That said, as a bit of a curveball the car has an Auto Authority MAF conversion, SSIs and a dual exit back box (I think it's a Dansk, but I'm not convinced it wasn't a homebrew dual exit conversion), because there is nothing like adding a few more variables into the mix, right?
The symptoms
When I got the car back in January, it had a polite "misfire" at idle - it would occasionally drop a cylinder, with a little fart. It also had a stutter at around 1800rpm-2200rpm on light throttle openings, but that would clear up at higher revs.
I haven't had much wrenching time since, but managed to get around to doing a minor tuneup (plugs, cap, rotor, fuel filter) and both plugs and fuel filter were well by its use-by date. The cap and rotor wasn't far behind. I was planning to swap out the plug leads but the aftermarket ones I got from the forum sponsor didn't quite fit as well as I had hope so I left them off for now. The car had Autolite Platinum plugs in it that were severely worn (gap around 1.1mm) so I hoped that those would be part of the issue.
Starting it up after the tune up, it idled a bit better - the "cylinder drop" went from about once 10-15s to maybe 20-25s between drops, but it was still there. Not as pronounced. I had the battery disconnected for probably a couple of months so I expected that the car was running on the baseline tune (assuming the Motronic does have any self learning capacity).
I did end up having to reseat the distributor cap (oops, my bad) and it ran fine when luke warm, with only a minor stutter around 2k +/-250 rpm at light throttle openings. Took the car out for a test drive and to thoroughly warm it up. During that time, I could easily drive around the 2k stutter by applying a little more throttle, and it would pull nicely to about 4500-5000.
About 20 miles I noticed that the RPM range of the stutter/miss was expanding, it now started at lower rpms and went up to almost 3k. It also started backfiring on downshift blips about every third or fourth time. Again, nothing massive, but an audible pop. Still could drive around the issue by stepping on the right pedal bit more.
After about 40 miles that, I noticed that I got an occasional hesitation even as far up as 4500 rpm under load so I backed off a bit. The overall problem did definitely get worse the more thorough the engine was warmed up - I've never driven the car when it was hot outside, so I didn't notice this behaviour until now.
No problem starting hot or lukewarm, the polite pop at idle is still there but didn't get worse.
Diagnostic plan
Based on reading the forum here, I suspect the car is suffering from one or more of the typical issues:
- Old plug wires. I'll chuck on the set I already have and see if there's any improvement.
- Possibly a bad coil. The coil looks OEM and I'd guess is at least 12-13 years old as I don't see anything in the maintenance history of the car that shows it's been changed.
- Battery isn't at its best, but was fully charged (it's usually on a battery tender) and holds a decent charge.
- Intake gaskets. I'll have to do the starter fluid/brake fluid check to see if I can find anything. I did already get a set of gaskets with my order of tuneup parts. If I'm pulling the intakes I'm also going to send the injectors off to get tested and cleaned.
- Sensors going bad. I haven't checked yet what kind of CHT sensor it has, nor did I check the O2 sensor or the crank sensor. Connectors in the engine bay look decent.
- Electronics issue re the MAF conversion - I have no documentation on the Auto Authority swap. My understanding is that it needs a new chip for the Motronic, but I have no idea what's in there and if it's been tuned to accommodate the SSIs and exhaust. That also prevents me from swapping in known good parts unless someone has a spare MAF and Motronic with the chip lying around
.
Does anybody see anything that's missing on the list above that I should check? Compression test might be in order, although I don't think any of my testers have the necessary adapters. Checking and setting valve clearances is probably also a good idea.
Last, if anybody has documentation on the Auto Authority conversion (especially around general setup, if the same ignition base timing still applies etc) and would be able to share a copy, I'd be very grateful.