Running rough (bump) but no CEL on dash or DTC in tech stream!
So I was driving across town to look at some running boards (RB) with AC on and OAT of 88 f. While sitting at a red light I felt a bump then another. It felt like AC pump cycling on then off twice in a row but with a kick, strong enough I felt. I'm thinking a possible issue with AC compressor. So I turned off the AC. Nothing happened again until after I'd parked engine off while I looked at the RB's. After loading up the RB's and again while stopped at a red light AC off, I felt a few bumps in a row again. RPM dropped a bit to ~500, then picked a bit high to 1K and steadied at ~600. I slipped into N and all seemed fine.
Since AC was turned off and bumps came back, AC is no longer a concern.
I've been a little concerned transmission fluid level was not set right, so now I'm worried. Thinking bumps may be coming from transmission or torque converter. So I decided to just let cool overnight, and do a transmission fluid check in the morning using mini VCI cable to hook up to tech stream to actually see ATF fluid temp to set level.
So this meant I needed to get my mine VCI cable working. It had been give me issues disconnect then not making connection sometimes asking for password a real PITA. So I uninstalled the software and reinstalled starting with oldest version, then bring update to newer until it worked. I'm now thing it's not a software issue, but a bad mini VCI cable. Tool issue
Anyway I did get my ATF temp read out after turning tech stream on and off repeatedly. It was necessary to shift between gears and load engine/transmission by holding brake as I raised RPM while in gear. Finally reaching the required ATF temp of 115 F (116.5 f) I pulled the transmission fluid level plug. Nothing came out so I added .4 qt and retested. Now ~.2> qt came out. So I was just a tiny bit low, certainly not enough to cause any damage to transmission from my test drives.
Found it:
While loading engine/transmission while in gear I spotted #1 cylinder indicating a miss-fire with the associated bump.
I suspect a weak coil. I didn't get a CEL or bad coil DTC, only a miss fire while under load.
To confirm it was a coil going bad and not some other issue like compression, fuel/air, ECM or wire harness issue I swap #1 coil with #3. Fantastic, miss-fire moved to number #3 cylinder now. I grab another used coil of the shelf and installed it.
In hindsight, I did have other indications a coil was going bad. i.e. fuel smell and oscillating sound during idle. What I first thought was possibly due to excessive idling, which is not recommended but necessary when working on engine, transmission and AHC testing. I noticed idle was not as smooth, more of a oscillating sound then steady as I like to hear, which became more noticeable with time. Also I noticed a raw fuel smell that was getting a bit heavy in the shop while idling over time.
If the budget allowed, I'd just put in all new coils at this point. But until a DTC states bad, they're considered serviceable. Same is true with A/F (front O2's) and O2 (rears) sensors. They are consider good until failure. But, IMHO they weaken with time and do not perform at peak through their entry life.
What do you say all new OEM coils and O2's now or wait for failure?
When to replace Ignition coils.