1989 Sierra K1500 Erratic Idle

Olexa21 · November 15, 2010

My truck will not idle right, it bogs down and goes back up and when its in gear and i hit it it takes a while to go and its still not full power. I have to hit the throttle just enough to move or it won't go. I have already changed the fuel filter, my cracked exhuast, plugs and plug wires, and cleaned the throttle positioning sensor, also changed the MAP sensor. I think i'm going to take apart the injecters today but I need major help

rodram · November 15, 2010

yes it will be good to check the spray pattern at the two injectors in the throttle body...

... but you also need to get a fuel pressure guage and check the fuel pressure..

... these TBI engines should have a fuel pressure of 9 to 13 psi ... and not much leeway on the bottom end... if you only have 7 or 8 psi fuel pressure this could be causing the problems you describe

Olexa21 · November 16, 2010

Well....Everything is back together and I took it to put 10 bucks in gas in and it ran good for about 20 minutes and then it started again, I was thinking Water in the tank and I could just put some of that heat from a gas station in the get rid of it, the only other thing I haven't done is replaced the whole computer system, which I have the parts for.

rodram · November 16, 2010

Ok.... you did not say whether you had checked the fuel pressure... but ..

based on what you say in the last post ... that it ran good for 20 minutes and then started to crap out ..

.. have you checked or replaced the Coolant Temperature Sensor (CTS), located on the top front of the engine near the thermostat...?

.. this is the sensor that monitors the engine temperature to signal the ECM (Engine Control Module) to vary the fuel mixture to the injectors.. if it is not working then the injectors will always be getting a good rich fuel mix, as if the engine was cold... and when the engine warms then the fuel mix is too rich and tries to flood out the engine all the time... maybe you get some black smoke too..

.. this is a cheap part $15-$20, and easy to replace (even I did my own..)

Don't rip into the computer yet .. there are still things to check that are easier and cheaper to replace...

Another point.. (don't know how you usually manage your fuel tank level)... but just buying gas at $10 a shot and running on the bottom 1/4 tank all the time is not good for the fuel pump, as the pump relies on the fuel in the tank for cooling

Olexa21 · November 16, 2010

I have not checked that yet but I will later on, I live in the Upper Penninsula of Michigan and even when it's cold like it is now it will bog way down and even stall in cold. It has this problem hot and cold. And I have not checked the fuel pressure.

GMstrong · November 20, 2010

How about your IAC valve?

ChRoNo16 · November 26, 2010

I had all the same problems with my GMC 2500 this summer before it died (Where it still sits in the "graveyard") Check your IAC, I can tell you now that if it starts and stays running your ECM is fine. I replaced mine when these issues started and it wasn't the issue.

Whens the last time your distributor was actually taken out and checked? When mine was taken out it literally disintigrated in my hands it was so worn out. For sure check fuel pressure, those kits are like 30 bucks, and in all honesty it may be as simple as a fuel pump.

ChevyTech77 · November 26, 2010

Check for the TBI base gasket sucking air with brake clean.

Check fuel pressure. 9-13 psi.

Check for loose reluctor wheel in distributor.

Check for weak EGR valve opening prematurely.

Check for weak ignition coil.

Olexa21 · November 30, 2010

I have the problem fixed, I had a buddy bring his computer over and hoooked it up and had 2 pounds of fuel pressure, replaced the fuel pump and it was good, but now I have one more question, my oil pressture guage stays at 30 until it gets warm and then after say 10 minutes it won't move from 0. Everything still runs fine and I checked the oil and put a quart in but that didn't help, and every once and a qhile my water temp will hit 260 for about a min and then it goes way down. I think its my guages but asking for help to be safe.

ChevyTech77 · November 30, 2010

Check each with mechanical gauges to verify correct cluster operation. That would be the quickest way.

Coolant temperature can be read off from the coolant temp sensor in the ecm data list also to compare.

GMstrong · December 2, 2010

Sounds like your thermostat.

Olexa21 · December 3, 2010

Yesterday it said a had 30 pounds like it should and it didnt srop at all, I think it just went bad, but one last question. I put 80 dollars in gas and filled it with 30 gallons. I'm at 300 miles on this tank and I can't see my gas guage(I have the orange strip with the gray bar guages.) and when its low it disappears. Do you think I should be getting more than 10 miles a gallon? It's a 5.7 liter 350. And it's also winter in the Upper Penninsula of Michigan. Got lake effect snow. After a quarter tank in just goes down so fast, maybe that guage is wrong? Or does my MPG just suck that bad?

GMstrong · December 3, 2010

Or bad injectors.

mjlimo82 · December 8, 2010

Your gauges act the same way mine do. The only exception is my sending unit doesn't work right, so a full tank in mine reads a little less than 1/4 of a tank. Otherwise, my temp gauge goes to about 230 before the t-stat opens, and my oil pressure gauge is always wrong. It'll even make the Check Gauges light flicker when it's warm and at an idle in gear. I think the oil pressure gauge issue is related to the connector.