1999 Sierra misfire

Rick Anzaldua · March 17, 2019

Have a 99 Sierra 5.3L 255K miles with a misfire that I cannot find the cause. Misfires only when it’s warmed up and at a stop doesn’t matter if a/c is on or not. Was misfiring worse, got better after I replaced plugs, (NGK), 8.5 mm wires, removed and professionally cleaned fuel injectors, and cleaned MAF. No check light. Mods include, shorty headers, true dual exhaust with X pipe, electronic fans, cold air intake, removal of EGR with block plates, tuned to accommodate no EGR. Thought I had found problem discovered three damaged spark plug wires. Replaced them seamed to help but still have misfire. The misfire does not turn off engine just has the slight miss. Any idea

CamGTP · March 18, 2019

If I had a scanner with me in the truck I'd be watching fuel trims to see how they are doing. It could point toward an intake or exhaust leak pre-o2 sensor.

Also double check fuel pressure, make sure it's meeting spec for idle and while under load.

carnau · March 18, 2019

Do you have access to a decent scanner? One that has a misfire graph, or counter? You need to know which cylinder is skipping. From there we can help you diagnose the problem.

For instance if the Number 1 cylinder is skipping.

Swap the number 1 and 3 plug and wire and see if the skip changes cylinders. If it does not -

Test the injector balance (a decent scanner and fuel pressure tester will be needed) If the drop is within specs

Test the compression of cylinder 1. It should have 150 - 175 non running compression. If it does

Preform a snap throttle compression test.

These are the steps you could do if you knew which cylinder is skipping. You can swap the injector test for the compression test. That's just the order I like to go in. You could also swap the injector with a different injector and see if the skip changes cylinder as well if you don't have access to the proper equipment. And I know you said you cleaned the injectors. That doesn't mean is has the proper balance.

I did a quick google search for a scanner that has a misfire counter and balance tester and this is the one that showed up.

Scanner - https://amzn.to/2ubIfDq

Rick Anzaldua · March 19, 2019

Thank you I believe that is what I will do. I’ll keep you posted.

Rick Anzaldua · March 21, 2019

Just finished getting a scanner test. Turns out cylinder 1 and 4 show a misfire. Scanner also showed a P0300 code. This code is not new. I do recall this code appearing but it would erase by itself. Also had fuel pressure checked. May have found the culprit. Fuel pressure was at 46 pounds while running. Would that cause a misfire? I believe that is below what is recommended. A new fuel pump is recommended. Where should the fuel pressure be? What is below acceptable?

carnau · March 21, 2019

37 minutes ago, Rick Anzaldua said:

Just finished getting a scanner test. Turns out cylinder 1 and 4 show a misfire. Scanner also showed a P0300 code. This code is not new. I do recall this code appearing but it would erase by itself. Also had fuel pressure checked. May have found the culprit. Fuel pressure was at 46 pounds while running. Would that cause a misfire? I believe that is below what is recommended. A new fuel pump is recommended. Where should the fuel pressure be? What is below acceptable?

Your fuel pressure should be between 379–427 kPa (55–62 psi). It could cause a skip definitely, but normally it will not just be single cylinders. Normally it will be across all cylinders. I still would replace the fuel pump though. I have said this on here a thousand times too. USE A GM PUMP! They last and you will not be installing a new one next year. And for only 150... It's worth it. https://amzn.to/2OgJ1Zb If you want to save $30 get a delphi https://amzn.to/2OldIw1 but for as cheap as they are... get the GM.

Is this a distributed engine or is it coil over wire? They had both this year.

Do you have the scan tool so we can check the counter and see which cylinder is skipping the most and fix it first so we know what's going on?

Rick Anzaldua · March 22, 2019

It is coil over wire. I forgot to mention I also upgraded to MSD Street Fighter coils. They are maybe a year old. I sure hope it’s not the coils. I got rid of the originals. The original GM pump is pricey. But I too have heard that’s the way to. I will keep you posted. Thanks for the great info.

Rick Anzaldua · March 25, 2019

I misunderstood, at $150 that is a bargain. However I decided to go with lifetime warranty and Delphi brand from Advance Auto. At $165.00 minus 25% discount. I just like the idea of being able return the product if it doesn’t fit or breaks. Well I installed the new pump. So far so good. The plug was the same as to the one I had so I thought well I’ll pug it in. Bad move. The plug was the same but the wiring was different. Yep I blew the pump relay. The 25% I saved I had to buy a new relay so there it went. Damn us DIY guys think we know it all. Reed the instructions for crying out loud. Oh well thank you again. In the event it starts up again I know where to go. Tons of help Thanks.

Rick Anzaldua · April 1, 2019

Hate to say but I still have a very small but noticeable misfire. Now the few things I have left to change is the fuel regulator. It starts right up with no problem. My question is does the fuel regulator function only when you turn the vehicle on or does it regulate the fuel pressure as you are driving? I ask because I’ve read a symptom of a bad fuel regulator is hard starts. The misfire is only when it warms up. Thank you in advance.

sefiroxx · April 2, 2019

FPR uses a spring to close to a default location creating a minimum flow rate. As rpm increases, either vacuum or solonoid control will close the valve (against the spring) to keep more fuel in the fuel rail.

Typical fail of the spring has no impact on startup, but could starve engine of fuel at high rpm. You need a OTC fuel pressure gauge to check pressure before throwing parts at it.

Typical fail of the diaphram introduces fuel though the vacuum line creating a rich condition at low rpm (idle)

Rick Anzaldua · April 5, 2019

Well I’m back to square one. Last thing I replaced was the fuel regulator. Still have a misfire. After I installed it, it did run fine. Parked it for a few hours then drove to a drive through burger place. The truck was on, a/c going everything was fine then it started to misfire. Then it turned off. Took a little while but started up. I could smell something. Not quite gasoline but maybe a mixture. Can’t really pinpoint. Drove home. Just for hell of it I changed the fuel filter again for the second time in a month. I checked the fuel pressure and again it is still at 48 max. Are you sure it should be at 55? As you can see on this thread I have changed the fuel pump too. Would a leak in the intake manifold be the problem? Running out of options here. I want to add when it is cool it runs really well. But after it reaches 185 degrees or so is when it will misfire. I do believe it is a fuel delivery problem that I have. I’m in Texas so maybe I can just get a horse if it can’t be fixed. Thanks for your help. I really don’t want to get a horse.

SC Tiger · June 25, 2019

On 4/2/2019 at 2:45 PM, sefiroxx said:

FPR uses a spring to close to a default location creating a minimum flow rate. As rpm increases, either vacuum or solonoid control will close the valve (against the spring) to keep more fuel in the fuel rail.

Typical fail of the spring has no impact on startup, but could starve engine of fuel at high rpm. You need a OTC fuel pressure gauge to check pressure before throwing parts at it.

Typical fail of the diaphram introduces fuel though the vacuum line creating a rich condition at low rpm (idle)

I'm also having a "random misfire" code P0300. I reset the computer several times and it would either throw P0300 or P0304 (Cylinder 4 misfire). I checked cylinder 4 and it was not firing, so I swaped things around until I determined it was the spark plug and replaced it, then replaced 5 more (deluge of rain came in so I had to stop).

Next step is to change the wires as well. They were all recently changed but it was with cheap stuff.

What I am getting is a bad misfire at idle and a noxious fuel smell out of the exhaust. Under load it seems to run fine and the power is there. The plugs were black.

Also, it struggles to start if it's been idling for a while or when it's been freshly shut off. But I let it sit overnight last night and it fired right up. The fuel pump is relatively new.

After reading your post, it sounds like the pressure regulator could be my issue. The truck sits more than it runs anymore anyway. It is possible the diaphram has ruptured.

Edit To Add - BINGO! Fuel Pressure Regulator was the culprit. Replaced it and truck is running MUCH better.

Edited July 1, 2019 by SC Tiger