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Computer?

Tiny
JUSTIN.BUTLER
  • MEMBER
  • 1997 GMC JIMMY
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 145,000 MILES
I have a 97 GMC Jimmy SLT with 145, XXXmi

I bought this vehicle with a seized engine and replaced it with a rebuilt engine and had it pass inspection. But would only pass while it was running because all readiness monitors would state "not ready" while the engine was off. While it was running on the other hand it 3-5 Monitors stated "ready"

I Am currently in NY and since it is november, weather is beginning to become much colder and as it gets colder, the Jimmy gets harder to start. I have ran diagnostics and each idea for the hard start and resulted inconclusive. Fuel Pump is new and turns on, Fuel Filter is clean, Fuel Pressure is at correct PSi, New Sparkplugs and wires, New Distributor cap, Just bought new battery 200 miles ago, new gaskets, All new sensors, Have correct coolant and is at correct level, Oil was just changed 400 miles ago and no metal fragments. It is to believe that possibly the memory within the ECM (computer) loses power when the key is turned to the "off" position and everything resets when that happens. Im not sure if the "stay alive" memory within the ECM was shorted when the previous engine was seized by the previous owners but I need someone to give me any other ideas to what it could be.

Also note: When turning over the engine attempts to start while turning over but doesnt cause you can feel something catching during the turnover process.
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Friday, November 26th, 2010 AT 12:09 AM

6 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
If you ran an engine scan and it shows nothing then it may be the crank sensor or cam sensor with to many ohms. Bu as you've said something catches when you try to start and it's got a rebuilt engine in it. Check the timing like make sure the rotor point to #1 when it 's at tdc as there is no way to check thetiming. Also your injector lines may have cracked in side your intake manifold as well as that will only show up as a lean or rich condidtion.
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Friday, November 26th, 2010 AT 2:00 AM
Tiny
JUSTIN.BUTLER
  • MEMBER
  • 8 POSTS
Everything for the distributor was lined up to #1 And lines are good. It has to be based on something within the ignition, electrical category and/or the fuel system. I've also changed out the ignition coil because I had a spare and no luck there. Since its was only getting harder and harder to start as it got colder im starting to look into the temperature coolant sensor and the Air charge sensor to see if maybe they arent working correctly and throwing off what the engine needs to start correctly in cooler temperatures.
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Saturday, November 27th, 2010 AT 3:51 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Usually the only time an ecm loses it's memory is when the battery is disconnected for a while. But as soon as you start it relearns again. On your second reply you didn't mention checking cam or crank sensor. These have to deal with ignition. They are also knownn to go bad and not set a code. The cam sensor is in the dist housing and the crank sensor is or should be in the timing cover by the crank. Unfortunately this may have had the problem when you bought the truck. Hopefully the gears on the distributor are not torn up or worn but I imagine as that was out you checked that. I just don't beleive that the computer is bad but stranger things have happened. One thing to check is the ground to the ecm as well as there should be one from the body to engine, battery to engine and ecm to body as that's where it gets its ground. There may be one from the battery to the body as well and make sure those are all clean as you only need to take them off and use a wire brush or sand paper to clean them. Other than that I would suggest getting it professionally looked at as I can't think of anything else to cause this. I am not familiar with the monitors as we don't use that stuff here where i'm at. But it would stste not ready while it's not running as the computer or everything else woldn't be working.
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Saturday, November 27th, 2010 AT 3:05 PM
Tiny
JUSTIN.BUTLER
  • MEMBER
  • 8 POSTS
Okay now Sorry for not explaining clearly. Now back in november I checked the dtributor cap and rotor. Both are actually brand new because I knew distributors are prone to going bad so I replaced it. So for the past couple weeks its been sticking around 16 degrees and is a lot harder to start. When I previously said "catching" I meant by some of the cylinders firing as the engine attempting to start. I looked unto the cam positioning sensor and its just fine. I tried the sensor in the Distributor also and thats fine too. Ive looked at the Mass airflow sensor, air charge, and temperature sensor. I feel that since some of the cylinders attempt to fire in the process of turning over I believe that its lacking fuel. Im not entirely sure if it is the fuel pump, but I did a pressure test and when it is not running it stays at 55 and dips at 40 when starting. Fuel filters still good
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Thursday, December 16th, 2010 AT 6:46 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Minimum fuel pressure for SFI injection 56-62 psiengine running. With CSi 60-66 psi your fuel pressure is low that's why it'shard to start and run. This can aslo set a code of cylinder misfire.
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Thursday, December 16th, 2010 AT 6:57 PM
Tiny
JUSTIN.BUTLER
  • MEMBER
  • 8 POSTS
What is odd about it is that once its started it runs like it never had an issue. So basically when starting it only has just enough pressure to get it to barely start but once started its fine? Also, no code has been given for a misfire.
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Friday, December 17th, 2010 AT 2:05 AM

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