About to give up 5.7 vortec

jake lloyd · January 22, 2020

I've had my 98 Tahoe for about 6 years now. It's always had a slight number 5 cylinder misfire when climbing steep grades at highway speeds. Not enough to feel but enough to set the engine light. The end of 2018 I was in Roseville for work "railroad" and drove up from down in the southern California. At that time the big fires in Paradise were going on and I couldn't get a hotel so in order to sleep in the "Hoe" I had to drive up the i80 all the way to the top of the pass "donner pass" and sleep in the parking lot at the rest stop to escape the smoke. The truck started misfiring badly up the pass and I just kept her going. It got down to 25 deg that night and next morning I woke up and drove back down the pass to Roseville to catch a train. No misfire problems that morning. I cleared the code for a number 5 cyl misfire with my scanner tool. Got back to Roseville a couple days later and went to start my truck. All it did was crank but no start. Spark tester showed spark but wouldn't start. No codes either. It would sometimes backfire while cranking. And puff smoke out of the throttle body where the air cleaner housing attached.disconnected the neg battery cable but never positive. Still wouldn't start after neg off then back on . I tried using my portable jumo battery. No start. I put a new crank shaft position sensor in. Still no start. Finally had it towed to a shop that said they were able to move it into the garage with a shop battery and put mine on the charger. Said they couldn't find anything wrong with it and gave it back to me. Drove it 4 hours back down the 99 freeway to my home and it was misfiring enough to set the check engine light and you could feel it. But now it was showing a number 2 cyl misfire. I have since changed the distributor with a new cap and rotor also and a new cam sensor and new plugs and wires and fuel filter. I've check the valve springs and none are broken. It still misfires all the time once the rooms get over 2k. It's real bad when going up a grade. Hard to keep speed except when the mil light goes solid and the timing must retard because then it sounds normal and picks up power but only for a few seconds before it misses again and the light flashes. I can Flor it from a stop light and pedal to the floor til I let off at 80mph and no misfire or power loss or engine light. Only once it's been driven enough to get hot and the rooms are over 2k does the po302 happen. I can even drive up a grade at 50mpg Wich is under the 2k and it won't misfire. But strangle sometimes doing 70 moh it will be misfiring for 20 mins then just stop misfiring for another 10 mins or so. It's so odd and it will give me a po300 random code before it gives me the po302 cyl 2 code. What I don't under stand is how for a couple years it was a num 5 cyl misfire and then after the no start issue it became a cyl 2 misfire??? Any ideas? I've heard injector talk before but it seems like a timing thing to me .

riverbanks · January 22, 2020

How even where the old plugs? thinking with backfiring,you have a failing spider,they got it in shop,cause it aired out from being flooded?

bronyaur · January 22, 2020

Did you set the cam retard (CMP) offset? I've had strange misfire gremlins when it's not set correctly. You'd think it would set a code if out of spec (+/-2), but I've seen it off as much as -14 and not set a code. Make sure you check it with the engine above 1500 rpm.

m&jtrans · January 23, 2020

Plugs n wires?

The Blue Mule

jake lloyd · January 23, 2020

The timing was set when I had a shop put in a new distributor couple months ago hoping that would fix the problem.

Now.i don't want to get too excited too quickly but I put another new crank shaft position sensor in today because I started thinking about how it used to get a num5 cyl miss up a grade and after it wouldn't start in Roseville it was giving me a number 2 cyl missfire above 2k rpm Wich was after I had out a new crank shaft sensor in while it was broke down at the train yard. So as of now I've driven 30 hard miles with the new sensor in running second gear at 60mph to get the rooms nice and high and just pretty much beat the hell out of it driving and so far the misfire code has not come back on. Possibly the crank sensor took a dump and that's why it wouldn't start and I killed off the battery or something to where it didn't nhave enough juice to start with the new sensor in it while it was dead until the shop put their test battery in and charged my old one. And that would explain why it has been giving me a number 2 cyl misfire for over a year and the number 5 cyl misfire it always had disappeared. .... I'll respond back in a few days or sooner if it throws a code again before.

jake lloyd · January 23, 2020

Just got done driving it hard for 60 miles with part of that being up grade at the tehachapi mountains. Check engine light came on while on the freeway flats doing 70 after slowing down from 70 to 60 then speeding back up to 70. Did that a couple times before I got the grade and got a po300 code then about a minute later it switched to a po302 code. Never felt the misfire like I have for the last year. Had plenty of power.

Going up the grade the light flashes a few times and the po302 went away and changed back to a po300. Once again no loss in power or loud misfire grumbling noise as before. After coming back down the grade and driving on the flat on the freeway and in town driving no codes or check engine light were thrown. So I'm going to guess that the previous crank sensor input in. Nov of 2018 was bad. This one might be bla little finicky also or needs to have the relearn procedure done possibly. Both of the sensors I have purchased are cheapo 11 buck ones from Amazon.