1990 olds 3.1 cold hesitation

[jburba]

Hello. Thought I was a decent "old school" mechanic,(currently building hybrid 1994 302ci SBC using '94 4.3 V8 crank for my 89 S10) but I'm stumped. 1990 Olds Cutlass Supreme SL owned by my grandmother with 82,000 actual on the car. I've kept the car up myself for years. Car always ran perfect, but recently developed bad hesitation or stumble ONLY WHEN COLD, and just off idle. Engine hot, runs perfect. No codes or MIL. First thought was fuel pressure; ckd perfect. Pump was bleeding off slightly so I replaced it anyway. Then ckd EGR; fine. Thought maybe worn TPS; scan tool live data showed it perfect. Thought maybe ign module; have tried 3 different ones along with 2 coil packs-no change. Thought maybe PROM or ECM; have tried swapping 2 different ones-no change. Knowing the 3.1's reputation for intake gasket problems, I changed it; no change. While intake was off I changed O2 sensor while access was easy and fuel press reg just because of it's age. I had already flushed the injectors, but while intake was off, connected fuel rail and triggered each injector 1 at a time; spray patterns looked good and no leaks. Seems to run rich at open loop; isn't that normal? Scan tool data shows integrator at 100+; block learn 120-130, no knock signal or timing retard. Fellow mechanic needs advice-please help.

[ProTech]

Seems like you've checked or replaced all the common issues on those engines!

When you had the intake apart, did you happen to ohm check the injectors? If not, you might want to consider pulling the plenum off and doing that. They should be no lower than 11 ohms across each injector, preferably 12 ohms. If one or more is low, that would be your issue and you should replace them.

Otherwise, the most common thing I've seen is an EGR opening or sticking when it shouldn't. This will cause an off-idle stumble, usually worse when cold. I don't think they had the digital EGR's on that year, so it could be an EGR solenoid as well.

[jburba]

Thank you Protech for your reply. I will ohm check the injectors. As for the EGR valve, that was one of my first checks. I actually made a block off plate to bolt to the plenum so I could completely eliminate the EGR as the cause. By the way, this car has the EGR valve that has the 3 seperate coils each one individually controlled by the ECM.
Also, I forgot to mention that (before I was able to see live data from the ECM) I replaced the ECT sensor. I also swapped out the map sensor and disassembled & cleaned the throttle body.

[ProTech]

Yeah, so you do have the digital EGR vavle which does not have a solenoid. That should be pretty easy to eliminate then, as you did.

Since you must have a scanner that can see live data, why don't you drive the car when it is acting up and watch the data stream for anything dropping out. That's usually the best way to find the issue. If you have freeze frame capabilities that will really let you dial in and focus on the problem that's happening. I'd still recommend doing an ohm check on those injectors, though.

[carriedi]

look at your TPS signal for glitches, what is your IAC reading? what is your fuel trim? you could be having a TPS that gliches cold because the cold temp raises the iac. iac acts like a choke and raises your ilde when cold. see if your fuel trim shows lean when cold. 90 is pre obdII. does your scanner have graphing capibilities? If you could look at the graphs when the hesitation happens it will give you a clue on what to look for. Scanner may not be fast enough to see a glitch in the TPS. You may need a lab scope for that.

[jburba]

THANK YOU ProTech! I checked resistance on the injectors and got these numbers: 12.4, 12.2, 12.2, 12.1, 11.3, and 9.1 ohms. Replaced the injectors and the hesitation is gone. You taught an old dog a new trick. Since the car ran fine hot, I never considered the injectors. Here is some info I found: car was very rich during open loop but when in closed loop the ecm could keep it leaned out enough to avoid setting a code. The live data supports this because the pulse width (pwm) was always very low 1.4 mSec and now is almost double that 2.1-2.3 mSec. Being an old school carburetor guy, I didn't know what the numbers should have been. Also, the Integrator value went from 100 (rich) to 128. This being an OBD1 vehicle the info from the ECM wasn't nearly what it would have been for an OBD2 so I can appreciate the added complexity of the newer system.
Also, I can offer a tip to anyone working on a late 80's early 90's GM W-body. If you experience a constant electrical drain to the point that it will discharge the battery within days and you can't find the problem, check the module for the KEYLESS ENTRY (located under the rear package tray next to the RR speaker). I had this problem 3 years ago, and even with my electrical knowledge it took me a while to trace it down. The module got so bad it actually opened the trunk with no one around. The funny part was I didn't even know the car had keyless entry because my Grandmother never used it!
Anyway, THANKS AGAIN ProTech!!!

[ProTech]

You're welcome! Glad you found the issue, I sure thought it might have been those injectors!! Thank you for returning and posting what you found, I'm sure it will help someone else too.