1992 Pontiac TS 3.8 GT

[Meridivs]

We just got another Pontiac TS with the 3.8 engine and a strange problem:
- when you start it sometimes the engine turns hard like the advance timing is altered (battery is in top shape)
- after you run it for a while it starts easily then, if you leave it for a few hours, it starts hard again

Any ideas what could be causing this on a 3.8 engine?

[LMP]

...I want to believe the battery is good...but anyway I'd like a voltage check while you start, when cold...and when hot...because I go for easy things first. Turning the lights ON and looking for how they dim while starting gives a good cue too. Even corrosion on battery posts can cause something like you describe.
After that, I'd verify for coolant level from time to time...even if the 3.8 is extremely reliable on that issue...water dipping in a cylinder at rest is a possible explanation, though improbalbe with that engine. I'm sure you are going to find other cues soon, as I understand you have the vehicle just for a few days at this time.

[Meridivs]

Thank you LMP, I was looking forward to your ideas!

Seems like a weak electrical connection or battery to me too (especially since this TS sat outside the whole winter until I bought it) but the mechanic working on it now already changed the battery and battery connections.

Are there any tell-tale signs of water in the cylinders?

Once the car starts it runs good, it even restarts fine if you don't let it sit for a few hours.

[LMP]

DO not neglect the cable terminal at the starter too....and even the starter itself, but run the car a little and see how it evolves. I would have described my problem almost similarly, 2 weeks ago but rather more dramatic...and in my case, it was the battery, and I knew it all along, 5YO and historically not a good one, .....finally would not keep a charge for a say, so hard to start...but OK once started if I went to store and little trips like that. New battery now.
Well, the water idea is a remote possibility and I'd wait for other signs to consider it more seriously. Notice that my 3.8 has ALWAYS depleted the radiator contents slowly, the overflow tank constantly going down....and I have NEVER determined positively where the coolant was going (although I suspect it sucks it at the intake manifold, but that is a series 2 problem, very rare with series 1) ....no sign in engine oil, no sign in transmission oil (because of coolant lines in the radiator..but at some point, I did get some transmission oil IN radiator...disconnected now and using an external transmission cooler), no leaks under the car, and no problem of any kind with heating or operation .......so.......hard to verify.

[Meridivs]

I was checking the TS Service Manual last night and it says something about the Crankshaft position sensor having something to do with start-up: it tells the computer which pair of cylinders are ready to fire first.

Any way we could check this sensor for proper operation?

Does it have a role once the engine is running?

Today I will try to see if there are any trouble codes set and come back with the result.

[LMP]

Cam position, not crank position, sensor tells which cylinder is on the compression cycle. Crank position tells when to fire....and notice that firing occurs on EACH turn , whatever the piston is on the compression or exhaust cycle. Cam position tells when to inject the fuel to the proper cylinder . Cam position sensor failure will pop a code. Crank position sensor failure is supposed to pop a code, but generally does not unless it fails totally while the vehicle is running....But the effect of a degraded crank sensor is generally just the opposite of your problem: it starts ok when cold...then will stop at the stop light 5 minutes later and will not restart before cooling down. However, one cannot rule that out completely and I have recommended that cars that age install a new CKPS and a new ignition wafer module to ensure reliability.

I think the 92 3.8 does not have an EGR valve and the means implemented to acheive emission rules are said to cause the burning of exhaust valve, a sure contributor of bucking at starting when the exhaust of another cylinder invades the faulty valve cylinder at an odd moment . A compression test would clear that option...easy on front cylinders, a headache on the rear.

[Jeffrv]

One place to look for is corroded/loose connections at the auxiliary battery post ( located beside the ABS assembly on the firewall).
Jeff

[Meridivs]

Scanned the car for trouble codes and there are none (the Diagnostics system is ok).

LMP, what are the chances for the cam sensor to fail without setting a code on these engines - did you had to replace yours by now?

Jeff, I will try to check the electrical connections on the firewall next.

Thanks for all your ideas!

[Jrs3800]

The crank sensor on a 3800 is a dual channel sensor... One used for spark( 3X's Pulse ) and the fuel pulse ( 18X's )... The Cam sensor is strictly used for a #1 TDC reference for true Sequential Fuel Injection...

Even if the cam sensor failed( code 41 ) the engine will still start run and drive running off the last known good fuel sequence... There would be a 1 in 6 chance that the engine fueling would be spot on...

Now like stated the battery, batter cables, and starter should all be checked... The 3800 is a quick start system... Should start with ease...