89 GMC 5.7L not running right . . . I'm stumped

Discussion in 'Chevy C/K Truck Forum' started by rbhankins001, Jun 12, 2012.

  1. rbhankins001

    rbhankins001 New Member

    The truck is a 1989 GMC 4x4 5.7L engine. It has about 69,000 miles on it. All original. It was bought brand new in Juneau, Alaska, which is landlocked hence the low miles for the age.

    I bought the truck about two years ago. At that time it has a problem the previous owner, and a few mechanics couldn't figure out.

    So, you could drive the truck so long as you accelerated very slowly (and I mean VERY slowly). It took many revolutions of the engine before it would fire off (but always would).

    Someone recommended the MAP sensor. I replaced this and there was a definit improvement. You could feel more power it seemed, but only sometimes. The other times it would stumble and hesitate, and I was back to very slow acceleration.

    The whole time, based on the symptoms, I suspected the ECT (engine coolant temp sensor). It ohmed out within spec. So I ruled it out.

    That was a mistake! It turns out I think the ECT was ok, but the connection between it and the wire harness was NOT. Replaced pigtail and ECT and now the truck has no check engine lights or codes.

    BUT, the truck still has something wrong with it. You still have to accelerate slowly or it will run rough.

    I don't know what else to do, please help.

    The town the truck is in has only 300 people, and no mechanics, you have to fix your own stuff . . .


  2. Chris Miller

    Chris Miller Rockstar 100 Posts

    It could be the EGR valve or EGR crud in the intake. It wouldn't necessarily throw a code.
    You might also try Seafoming it.
  3. K15 Blazer Guy

    K15 Blazer Guy Member 2 Years 100 Posts

    first, check the spark plugs and make sure they are all burning clean with no buildup or discoloration.

    also, just look at the injectors of the throddle body when running. may be bad fuel pressure or filter. rev it up a little too to see if they faulter at all.

    Also, try a can of throddle boddy cleaner, and a sea foam treatment . (one can poured into the intake, and 2 cans in a full fuel tank)
    that will be cheapest way to clean up a long sitting engine and its internals; like vale seats and EGRs.

    then let us know what that search shows :great: good luck
  4. rbhankins001

    rbhankins001 New Member

    So I'm going to be heading back out to remote Alaska and mess with this truck. I really have to fix it this time so I can launch a boat.

    I appreciate the advice so far.

    I think the first thing I'm going to do is check the fuel pressure. Does that seem like a good start?

    I'm wondering (only a guess, I'm no mechanic) if the fuel pump is making pressure but not volume?

    It is so odd to me that if you lightly apply pressure to the gas pedal and take your time and accelerate slowly it will run all day like that (all summer last year) BUT if you press down any more it will stumble and run like a turd.

    There are no engine codes.

    Your advice is greatly appreciated, the truck is in the middle of nowhere and I have no choice but to diagnose and fix it myself.

  5. geo1

    geo1 Member 100 Posts

    fuel pressure,on that model i cant remember,is the fuel return line pliable,im thinking it is,right after the pressure regulator,throw some visegrips on there,not to tight,but enough to restrict some,thats a nocost diagnostic aid,if there is improvement then you have low fuel pressure,on a whim,has you or anyone done a vacuum test,one to see how the cat. is,two just to monitor intake vac.
  6. rbhankins001

    rbhankins001 New Member

    I believe the pressure should be 9-13 psi from reading on the www. I am suspicious about the cats too, with the bad connection on the ECT it would have been running really rich right? If you tell me how to do the vacuum test I will do it. I don't mind spending a little on some tools. I have to fix it myself.

  7. geo1

    geo1 Member 100 Posts

    9 to 13psi is carbed motor,youres should be 40,i think,you are t.b.i,right?no tool way for vacuum test is,pull off vac. line at e.g.r.,hold finger on line,line should stay on finger if good vac.,dont rule out e.g.r. either,plug that vac.source and see if there is a diff.also,do both of these tests at operating normal temp,one other try is disconnect a sensor at a time and test drive,remember comp. has base paramiters built in,i.e. 14.7 to 1,air to fuel ratio,do m.a.p. first,electric connector,reconnect then do vac.line,if no change go to t.p.s.,test again,no change,disconnect c.t.s,all you will do is put comp. in open loop,check engine light will be on,hopefully,well get to that later
  8. geo1

    geo1 Member 100 Posts

    any news on truck?i had computer problems,i think im back.

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