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Will not stay running

Tiny
TOEDRAG
  • MEMBER
  • 1999 GMC YUKON
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 185,000 MILES
The vehicle was running fine but after a short period of inactivity, now it won't start. The engine cranks over and I can smell fuel, the fuel pressure is 55 PSI. I can't check for spark by myself (help) and I cleared an O2 sensor code indicating a rich mixer. I replaced all of my ignition system components except spark plugs but all eight plugs don't normally fail at once so I don't think the spark plugs are a concern. When cranking, the engine never fires or starts and shuts off (security key) although it has done this in the past. Now it just cranks and the split second I let go of the key it coughs like a light backfires through the throttle body. I normally do my own maintenance because I'm a certified aircraft mechanic but before I start replacing catalytic converts and exhaust systems, I would like some feedback.
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Wednesday, April 14th, 2021 AT 2:56 PM

10 Replies

Tiny
HARRY P
  • MECHANIC
  • 2,291 POSTS
To start the vehicle, fuel pressure needs to be at least 60 PSI. 55 won't do it. So if you can only get it to 55 with the key on, engine off, then that's your problem. Frankly, this is common. I had a 2002 Blazer (Same pump) that wouldn't start unless I turned the key on for a couple of seconds to let it prime the system, then off and on again, and then cranked it. After a few days of that it gave out altogether.

In short, my money is on the pump. Again it's common. Over the last 7 years I've owned 3 Chevrolet Blazers and 1 GMC Jimmy. I've replaced at least 5 fuel pumps in them. The third time I did one on my 2002 I got sick of it and cut and access hole in the floorboard. After that the entire job only took 15 minutes.

As a test measure, try spraying some starting fluid into the intake and then crank it and give it a little gas. I bet it starts. It might even hold idle and drive.

Another test (more of a backyard trick) is to talk the fuel tank while someone else cranks it. Sometimes that jarring effect on the pump would running will give it a little boost and get it over 60 PSI. But (1) you need a helper or a long striking object and (2) even if it starts it's not fixed, just well enough to get home or a shop.
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Wednesday, April 14th, 2021 AT 6:13 PM
Tiny
TOEDRAG
  • MEMBER
  • 6 POSTS
The gauge I have for checking the fuel pressure is old with bad resolution so the PSI could be 60. I also just replaced the fuel pump 11 months ago. I tried gas down the throttle body last week and starting fluid last night and it didn't make a difference, still no fire. Any other ideas would be appreciated.
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Thursday, April 15th, 2021 AT 3:34 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,070 POSTS
This sounds like the crankshaft angle sensor, when you turn the key to the on position without cranking the engine over can you hear the fuel pump run in the tank for 5 seconds? These guides should help us fix it:

https://youtu.be/dCjmRL3p4Cs

and

https://www.2carpros.com/articles/car-cranks-but-wont-start

Also, lets run the codes to see what come up.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

and

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please run down these guides and report back. Can you please shoot a quick video with your phone so we can see what's going on? that would be great. You can upload it here with your response.
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Thursday, April 15th, 2021 AT 5:23 PM
Tiny
HARRY P
  • MECHANIC
  • 2,291 POSTS
I agree with Ken. Could be the crank angle sensor. Run the codes to see what we get. There may be something in there even if the light isn't on.

If that doesn't do it, then if still like to be absolutely sure that we have enough fuel pressure. You might have to go borrow a gauge from AutoZone or whatever store near you that has a loan-a-tool program.

I replaced a fuel pump in one of my Blazers 3 times within a year. That was when I decided to create an access hole. The second and third times were within a couple of weeks of each other.

Essentially, after market pumps are hit or miss.

Let us know what you find
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Friday, April 16th, 2021 AT 3:57 AM
Tiny
TOEDRAG
  • MEMBER
  • 6 POSTS
I was out of state for an extended period of time, hence the delay, but I'm back in the saddle trying to troubleshoot my Yukon. Since my last update, I've verified my fuel pump is working well at 64 PSI. I've changed my spark plugs and crank sensor. Wires and coil pack are new as well. Still just cranks and every once in a while it fire's workout starting. I checked for spark and it's weak at best with a low yellow color and definitely not blue. One spark plug was very wet but the other 7 where perfect in color. No codes other than the original o2 sensor and still no luck. Any ideas on why the weak spark?
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Tuesday, June 8th, 2021 AT 3:37 PM
Tiny
TOEDRAG
  • MEMBER
  • 6 POSTS
I should probably add, the catalytic converters are original. The flange between the catalytic converters and muffler was leaking for about a year and I finally welded the flange so it didn't leak, about 5,000 miles ago. I would like to understand the weak spark issue and rule this out before I start with my exhaust system.
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Wednesday, June 9th, 2021 AT 12:57 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

The first thing I would do is check the compression. Make sure you have 140-160 pounds. Low compression will cause this.

https://www.2carpros.com/articles/how-to-test-engine-compression

Before you go to the converters, disconnect the head pipe on both sides and see if it starts. If it does, the catalytic converters are clogged and the engine cannot breathe. I attached pictures of the removal of the catalytic converters for you below.

https://www.2carpros.com/articles/catalytic-converter-replacement

Also, I saw nothing about the ignition module. That is what controls the spark to the plugs. That would be something I would replace. I attached the procedure below for you.

Roy

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Sunday, June 13th, 2021 AT 4:04 AM
Tiny
TOEDRAG
  • MEMBER
  • 6 POSTS
Roy, my compression was 60 to 90 PSI on the first three cylinders checked. This leads me to believe my timing chain has jumped a tooth. I'm the original owner and I know anything is possible, but does this seem realistic with 185,000 miles on a well maintained 350.
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Monday, June 14th, 2021 AT 3:18 PM
Tiny
TOEDRAG
  • MEMBER
  • 6 POSTS
BTW, thank you for all your expertise Roy.
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Monday, June 14th, 2021 AT 3:19 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
You are welcome.

185,000 is beyond the recommended time to change it. 150,000 is when it is recommended. They put the cheap set when they made the motor.

When you replace it, I would use a double roller chainset. The original is not a double roller. The double roller became the standard around 2000. There was no recommended tie for changing that. They just state to replace it at the engine overhaul.

I attached the procedure below for you.

Roy
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Monday, June 14th, 2021 AT 4:25 PM

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