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Old 12-26-2011, 06:44 PM
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1989 C1500 Drivability Problem(Surge/Stutter)

Hello all. I have been fighting with an intermittant surge and stutter in my truck. Engine runs fine for a while(sometimes 15 minutes, sometimes 100 miles) but it will always develop this problem eventually. The surge /stutter occurs at any engine speed, but always at light throttle imput like just trying to keep up with 35-50 mph traffic. Runs at WOT fine. Accelerates fine. Decelerates fine. Idles fine. Wont run right under load. Some background: 1989 chevy c1500, 5.7L, freshly rebuilt. Bored .030, stock replacement cam, stock TBI, replacement cat convertor, knock off flowmaster single in dual out muffler, 195 thermostat. Throttle body was rebuilt with new ac-delco kit and same part # injectors. Distributor was replaced with ac-delco replacement. Remainder of ignition is new. EGR valve was replaced with ac-delco part as was solenoid for it. Basic stock rebuild of long block by local machinist I have used much in the past. Transmission is a getrag 5-speed, truck is 2WD. Trans was rebuilt and clutch parts replaced, including flywheel. I had the harness out of the truck to paint engine compartment and to check it over for bad spots. None found. Truck has not run right since it was put back together. Reason it was rebuilt was that it was just flat wore out, 187000 miles(weak or broken rings). Fuel pressure tests at a steady 11psi idling or driving. Fuel pressure does not fluctuate when problem occurs. Truck runs at 195-200 deg. according to my Snap-on scanner and my laser temp probe. Scanner indicates all data is good at idle and data stream while driving is all good. Does not store any codes. I guess what I'm asking is does anyone have any thoughts as to what I'm missing? What would you check next? Thanks in advance and sorry so long. Trying to be thorough.

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Old 12-26-2011, 07:26 PM
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Could be a bad spot in the TPS. Sounds that it is related to throttle inclination as you indicated as to where / how it occurrs.
Could be an ECM or Distributor problem also.
distributor wear can effect the VR signal at certain rpm's , also an ECM can have internal issues.
One other thought would be any computer related wiring laying anywhere close to the Ignition wiring, causing interference with low voltage circuitry that is sensitive to basic operation.
I would check the plug wire routing and computer harness first.Keep them apart as much as possible.
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Old 12-26-2011, 07:35 PM
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Thanks for the info. Distributor is new and in all aspects seems to be tight and servicable. I will double check wire harness routing. I have another ECM, but it has a different prom. Same service number ECM , so i could switch proms and ECMs correct? I would not think problem would be prom related without it showing up on scanner, or could it be? I have heard of internal problems with the ECMs and will try this. Do you think an "auxillary " ground off of the ECM case to chassis or frame would be a good idea? Thanks. Also I have "sweep tested" the TPS with a digital volt meter. It seems okay but I guess it would if it only happens at random.
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Old 12-26-2011, 07:56 PM
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The ECM grounds through the wiring harness and doesnt need a "case" ground.It cant hurt anything though.
Check the prom number in the alternate ECM to see what it is for. It could be for the same engine or not,same gear ratio , or not, or it could simply be an updated prom that was designed to clear up an Idle problem or some other minor tune that was a problem when the truck was newer and still dealer serviced.Prom numbers change for many reasons.
If the alternate ECM prom combo is correct or real close (say for 305 instead of 350) it will run plenty good enough to drive it so as to indicate whether or not the issue exists in the ECM.
My brother put a 350 in a 88 astro and put TBI on it. It ran fine with the original ecm and prom. The original motor was a 4.3 V6 . It actually ran pretty darn good.
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Old 12-26-2011, 08:01 PM
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This may help ID your unit
http://www.exatorq.com/ludis_obd1/idxref.html
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Old 12-26-2011, 08:27 PM
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Okay, thanks for the help. The alternate prom (and ecm) is out of the same year truck with a 5.7 and 700r4. Known good(or atleast was), took it out myself from running truck. My truck has a 5-speed, don't know if this will make a big difference as far as diagnostics are concerned. But thanks , I have some more things to look at now.
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Old 12-26-2011, 08:44 PM
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The ECM from the truck with the 700r4 will require a signal input from a shift lever sensor to tell the ecm the truck is in gear. With a standard you wont have that. The timing curves between the 2 computers will be noticeably different due to that alone. The computer/prom from the auto trans model may seem sluggish.
However it could still provide you with a usable test result if the misfire, bucking, jerking goes away. If it does, then swap the PROM over to the replacement ECM and drive it again to see if it is still OK. If it is, That would indicate the trouble was the ECM and not the prom.
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Old 01-01-2012, 11:29 AM
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Quote:
Originally Posted by 40 grey
Okay, thanks for the help. The alternate prom (and ecm) is out of the same year truck with a 5.7 and 700r4. Known good(or atleast was), took it out myself from running truck. My truck has a 5-speed, don't know if this will make a big difference as far as diagnostics are concerned. But thanks , I have some more things to look at now.
Just swap your chip to the new ECM... actaully the 1227747 has 2 removable chips, one is limp home for catastrophic failure, the other is the one the engine runs on. Just swap both.
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Old 01-01-2012, 11:37 AM
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After re reading this post it occured to me that what youre describing sounds like a torque converter lock up shudder and not an electronic problem.
depressing the brake pedal unlocks the converter to aid in braking the vihicle. Why I mention this you ask?
If you want to determine the torque converter is the problem, drive the vehicle, you will need to use both feet. Run the gas with your right foot, when the problem starts acting up, lightly touch the brake so as to disengage the torque conveter, if the problem quits when you touch the brake then the issue is with the torque converter.
If that is the case,a fluid change would be a possible fix.50/50, either it will or it wont.Its the least expensive thing to try.
If that doesnt fix it, you may need to replace the torque converter.
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Old 01-01-2012, 06:08 PM
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Standard tranny has no torque converter.

700r4 does not have the shift position/neutral safety switch. You're thinking of the later 4L60e.

1989 700r4 should have the VSS. Not sure how the signal from the 5 speed is done.
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Old 01-01-2012, 06:14 PM
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Quote:
Originally Posted by 123pugsy
Standard tranny has no torque converter.

700r4 does not have the shift position/neutral safety switch. You're thinking of the later 4L60e.

1989 700r4 should have the VSS. Not sure how the signal from the 5 speed is done.
Hee Hee, took my eye off the ball pugsy,
what are the odds that the knock sensor is acting up?Just a thought.
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Old 01-01-2012, 07:09 PM
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VSS on a lot of Chevy trucks is in the speedometer.

700r4 does have a park/Neutral switch and also a brake switch, when brake off TCC will lock up, when brake on TCC off. HTH!
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Old 01-04-2012, 09:18 PM
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Thanks to all for their help and suggestions. After further testing and retesting, the EGR valve was found to be intermitantly sticking open when solenoid commanded it open(cruise over 40mph). Oddly enough it would manually test okay, held vacuum, and pintle moved freely off the truck. When on the truck it would sometimes not seat or open freely after it was hot. New one(ac-delco) is coming tomorrow. Hopefully this is the problem. ---

---BTW, it ran pretty good with automatic ecm and prom. Low idle(500rpm), and shift light on steady were only observed problems.-----
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Old 01-04-2012, 09:24 PM
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and make sure the passege in engine is clean/not plugged!
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Old 01-04-2012, 09:43 PM
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Will do. Intake was hot tanked with the rest of the engine parts at rebuild, so I'm pretty sure it's clean. But will check at install. Thanks.
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