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  1. #1

    Default 1994 Suburban 5.7 Idle high or low, sometime's dies and affects shifting

    Hello everyone. I have a 1994 suburban that has intermittently been doing some strange things. It seems to mainly do it once it gets warm. Once and a while, it will idle high(1000-1500 rpm) and when it does this, it seems to shift really hard when shifting into gear but it also will not shift down into 1st even manually. It kinda feels like the trans is slipping when it does this but it may feel this way cause it's starting in 2nd. Sometimes it will go the other way and idle very slow(like doesn't even make the factory tach move) and when it does this, it seems like it's running very rich cause it's smoking black. It sounds like the timing is retarded a few degrees. When it's doing this, it does not affect the shifting. It was throwing MAP sensor high and low codes, but haven't been able to get it to set any codes lately. It has a new evap purge solenoid, egr valve solenoid, egr valve, tps, idle air control valve and a new distributor. It has me stumped. Any help would be great. Thanks

  2. #2


    How many miles do you have on it?

    Have changed your O2 sensor?
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  3. #3


    The chassis has 239,xxx on it. The engine has been in for 5k. I have an edelbrock performer tbi intake manifold on it with an rv cam, shorty headers, no cat, and dual flowmaster 40 series. I was thinking about it last night an i'm thinking it might be the coolant temp sensor. When it idle's high, it is about the same as when the sensor is unplugged. I am going to mess with it either today or tomorrow morning and going to try driving it with it unplugged and see if it reacts the same.

  4. #4


    From what i can tell, it's nothing with the coolant temp sensor. I unplugged the o2 sensor to see how it does. I had another 94 burb that when i had the true duals put on, they cut the wires going to the o2 sensor. I never had any issues with that one.

  5. #5
    Sr. Engineer
    Join Date
    Dec 2011
    11-22-boogie woogie avenue


    Sounds like the tps/throttle position sensor.

  6. #6


    That's what my next step is going to be. I replaced it back in november when i dropped the new engine in. I messed with it a little today. Checked for vacuum leaks again and the little things, and for any loose grounds or other connections. Let it idle for 1 hour outside and then went and drove it. I made it about 8 blocks, and was slowing down for a stop sign and it shut off just like you turned the key off. I hooked up my code reader and there were no codes at all in it. I cycled the ignition and it started right up. Continued to drive 6 blocks and sat in the drive thru for 20 minutes and drove back home 14 blocks and it never acted up again. I really hate intermittent stuff like this... Everytime it does it, I never even have a chance to get the hood open and it comes out of it.

  7. #7
    Sr. Engineer
    Join Date
    Dec 2011
    11-22-boogie woogie avenue


    Had a intermittent fuel pump do that . Drove me crazy quit then start right back.
    Sometimes quit had enough time to raise hood check a few things start right up.
    Only way i caught it was to watch fuel gauge real close seen needle get erratic right before it quit.
    Took me a week to figure that one out. Changed pump good to go.
    Timing or the maf will cause your symptoms also A cts will also .
    If cts bad ecm will go to limp mode.
    Last edited by dipstick; 05-20-2012 at 07:58 PM.

  8. #8


    I was almost thinking fuel pump to but I just remembered I had an 88 suburban to and I had replaced the tps on it with one from oreillys and it lasted about 5 months and it started idling weird. Got that one warrantied out and has been fine since. That was 6 years ago so I'm going to try this first before I go to the pump. I'll update tomorrow after I have the new one on.

  9. #9


    Ok. I finally got it to throw a code. DTC51 prom error. From what I have read on other threads, I'm going to pull the ecm out and remove the prom and reinstall it and hopefully it goes away. Is there anything else to check? and if I need an ecm, where's the best place to get one?

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