Stalls and Bucks

[dckingan]

Problem - Engines starts and idles nicely 650-700 rpm. After warming up it will sometimes stall when coming down to idle at a light or sign. It will restart but goes crazy running 2500-3000 rpm until it recovers and comes back down to a normal idle. Also when decellerating with foot off the gas pedal and reaccellerating it hesitates and bucks like the engine had stalled and restarted. I have checked and repaired all vaccum leaks, rebuilt all the injectors without success. It does appear the idle screw has been tampered with, if there was a plug on the adjustment screw it is gone now. Any suggestions would be greatly appreaciated.

Thanks in advance,
Dennis

[gbrtng]

Any SES light? If so, read the code(s) and report back.

[sammy]

you might want to do a fuel pump ck , sounds like what my 90 did a while back .it was the primary fuel pump starting to go out .

[dckingan]

Thanks gbrtng,

No SES light but will check for codes in case light is not working correctly. SES does come on and go out after starting. Car is currently in hibernation but want to put together a "to do list" with your and others ideas when the weather warms up.

Dennis

[dckingan]

Thanks sammy,

Will check out fuel pumps when car comes out of hibernation.

Dennis

[tomtom72]

Hi Dennis, pull the Fuel Pressure regulator line and see if there is gas in it...that's the down & dirty diagnostic that may give you a fast answer. You could also do the KOEO fuel pump test in the garage just to see what the pumps are putting out. The test is in section 6 of the FSM. I know on my 90 I replaces my pumps just because they were 18 yrs old.

May I ask who rebuilt your injectors? Usually our GM injectors don't respond well, specially the 90 & 91 injectors, to rebuild attempts.

Tom

[dckingan]

Hello Tom,

Thanks for your response and I have also looked at some of your other replys on hesitation. I have a service manual so will read up on on the procedure and dive into the pumps as soon as the weather warms up.

I had my injectors rebuilt by Ignyte, 1517W North Carrier Parkway, Suite 130, Grand Paairie TX 75050, 972 408 0944. www.ignytparts.com I forgot the guy's name but he was very helpfull and said he had a lot of trouble with a couple of the primaries but eventually got them all working. I think the early injectors were not necessarly friendly to ethenol so have been using an additive since the rebuild (November of 2009, $39.95 each). The stall and buck started last year so don't think it is related to the injectors.

Good luck,
Dennis

[tomtom72]

Hi Dennis! Okay so I'm assuming that you have an early car? Yes the alcohol eats the insulation on the coil windings & they just stop working.

The Idle screw didn't have a cap on my T/B. Usually no one knows it's there, on the underside out of site as it were, so the screw usually escaped people messing with it. If you know it's there, you know not to mess with it!

Your symptoms seem to be temp dependent to a large degree. Do I read you 5 by 5 on that? This is the way I would approach this issue. If you have a scanner, hook it up and drive with a co-pilot until the car acts out and then look at the displayed data. Me I work alone so that don't work too well, trust me on that one! If you're in the same boat then hook up the scanner and wait for the symptoms to appear or induce them if you can & observe the scanned data.

In any event I would read the section on Intermittent troubleshooting in section 6. JMHO!

Tom

[dckingan]

Hey Tom,

Thanks again. The ZR1 is a 1990, 1505 of 3049. I'm not sure the problem is 100% temperature related. It occurs after reaching operating temperature reguardless of ambient, which doesn't take long. I can predict a stall by putting in the clutch and watching the tach while coasting to a stop. If the rpm steadly drops and reaches idle (I understand idle doesn't occur until the ECM sees 0 mph) it will not stall. If the rpm drops to 400 or 500 and recovers to 1000 or 1200 it will drop again and stall. Sometimes it will cycle a couple times. If I restart before stopping with the clutch it goes crazy to 2000 or 3000 until it settles down in 20 to 30 seconds. Same thing if I restart with starter. The buck occurs if I take my foot off the accelerator and re-accelerate. Does not seem to occur if I don't take my foot off.

Anyway, off to Florida next week where I will study Sec. 6 of the manual. When I return mid-April I will wake her up, reset the IAC, check the TPS for 5.4 volts, check the pressure from the pumps, check for codes and have a friend who has a sanner watch it while I drive. Are there particular parcameters we should watch? If all this fails I will have her ( we call her Christine) exorcised.

Thanks for your patients,
Dennis

[tomtom72]

Dennis, no problem. If you get a chance look at the hose that connects the MAP sensor to it's vacuum source at the rear of the plenum. Something is giving the ECM fits and I suspect it is an "input" signal.

Okay, humor me and my 1/2 a$$ed mechanical non-existent diagnostic skills! LOL I'm thinking that some blow by oil got from the plenum into the MAP hose, or the hose is collapsing under high vacuum conditions ( like closed throttle coast down?). Look to see liquid oil in hose or even getting up into the sensor itself. Or that the hose has become soft from oil and is collapsing under high vacuum. Either one would throw off the Hg reading to the ECM and proly mess up the fueling? Does she do this miss thing at WOT way up in the 6k rpm range also? Okay this is the only other off the wall thing I can think to write down here. Otherwise, yea the TPS voltage and look to see that the TPS voltage is responding from 0% to full wot. Yes to the IAC, and I have to vote maybe on fuel pump performance, but check the pressure regulator hose for gas.

See you in the spring, unless I figure something out that is worth while typing here!


Tom