I picked up a very clean 1988 Firebird Base with the 2.8L V6, with 40K original miles in the dial. Guy I bought it from had it for a year or so and did a full tune-up, but since I bought it I notice that when idles at a light or in park the engine seems just a little rough. It is not fluctuating a whole lot of RPM and idles around the 600-800 range, but it is noticeably shaking a bit . Always starts without issue and so far has never stalled. I am just worried it is a sign of things to come. I haven't driven a car from this era for a couple decades, so I don't remember how smooth these things are supposed to be. I have changed most of the common things you swap for a smoother idle, but they haven't really changed anything. If it is not stalling, should I even be worried until it does?

 

Well, you would except it to idle perfectly if everything was as good as factory, right? I’m in the same boat as you though. New injectors, new plugs, new plug wires, new distributor, new IAC, adjusted TPS, set timing (advanced), and it still fluctuates a little. Runs and drives great.

Maybe a vacuum leak? These hoses are quite old by now. I’ve chalked mine up to a vacuum leak that I just haven’t found. Sorry I wasn’t more help, I’m sure one of the regulars will chime in with some more helpful stuff

 

The 2.8s were far from smooth even when new!

I wouldn't worry.

 

Yeah my sister had an '86 with the 2.8 back in the early 90s. I drove it some when I had my drivers permit, LOL. I don't remember it being a very smooth engine even back then.

Oddly, that engine seemed much smoother in front wheel drive cars like the Beretta.

 

it probably WAS smoother cause those had the FWD version Seies II design with the aluminum heads.


My 2.8 V6 6000 was and always was smoother than any RWD V6

 

They have a rythmic "shake" at idle because of the 123456 firing order.

 

Sorry for the minor necro but this is what I am investigating on my own car currently. If its too retarded it shakes and rumbles at an idle. Mine likes to be incredibly advanced, however the downside is it hunts a lot (swings +-75rpm) as its going for o2 crosses. Hopefully soon I'm going to tack onto the idle code to make it be a lot more hesitant to adjust the mixture unless it absolutely needs to after settling.

 

I can’t speak too much for the average V6, but I have my timing advance as far as possible without spark knock and have the same situation. Hunts by around 75rpm, sometimes (not all the time, seems random). I tried retarding a bit, but with the bigger injectors I have in there and my lead foot it just murders the MPG. Throttle response also gets crappy too.

The cars been running fine apart from that so I’m ignoring mine for now. Dumb question but have you checked for a vacuum leak? That’s what I’ve chalked mine up to.

 

I have not specifically looked for a vacuum leak. Watching the injector pulse width and integrator I can see it adding/removing fuel perfectly in tune with the hunting, so I had just figured it was that. eventually here like I said I plan on more or less fixing the mixture at an idle because everything else is more or less stagnant. (iac, timing, load, temps, voltage, tps, etc) while the integrator keeps chasing perfection lol.

 

Quick screenshot. I had typed out a longer post and then my computer restarted. Oxygen sensor is self explanatory. Bottom graph is the only thing that matters. Cyan is integrator, blue is pulsewidth, red is LV16, green is rpm.

It seems to me (though obviously I am biased in my favor) that the rpm swings match the integrator waving back and forth, and you can see a correlation between the rpm peaks and the O2 crosses (blue on the top graph)