Code PO300, popping sound coming from exhaust

STEVE BLACK

MEMBER
2000 CHEVROLET S-10
4.3L
V6
2WD
AUTOMATIC
161,000 MILES

My truck became hard to start. When there was rain/moisture no cranking. By end of day if humidity dropped it would crank. When it did, the muffler exploded. I had muffler installed. Replaced all plugs, wires, dist and cap/rotor button, all 3 oxygen sensors. The engine code reader says PO300 "random misfire". The next phase of testing revealed a bad EGR valve. I replaced it. Reset ECM per specs. Wait a day to clear mind as well. Cranks but still the "popcorn" or "shellfire" sound from tailpipe. I've made sure there are no leaks around 2nd new muffler, exhaust manifold gaskets, etc. The cat was deleted long before any of this started. Drove 15,000 miles post catalytic converter removal without probs. Have added Seafoam to gas tank. Checked all vacuum lines, installed new air filter, cleaned MAF sensor. Regardless of my actions, still a PO300. Please advise. Thanks

6 Replies

JACOBANDNICKOLAS

MECHANIC
109,644 POSTS

Hi,

Are they small backfires from the exhaust? If so, it could be a rich fuel mixture. Take a look through this link.

https://www.2carpros.com/articles/engine-backfires-while-running

I noticed you are a mechanic. Have you checked the fuel pressure? Also, do you know what the fuel trims are?

If you haven't checked pressure, start with that. If it is within spec, using a live data scan tool, see what the short-term fuel trims are and let me know.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

If the pressure and trims are good, the next thing I would suggest is to check compression. You may have a few exhaust valves leaking or burnt. Here is a link showing how it's done:

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

When checking, the minimum compression in any one cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read below 100 PSI.

Let me know what you find or if you have other questions.

Take care,
Joe

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Sunday, May 16th, 2021 AT 9:04 PM

STEVE BLACK

MEMBER
3 POSTS

Thanks for your response. Yes, I have a scan tool and have checked all the data you mentioned. My problem was a worn distributor with too much up and down slop. I replaced with a new distributor that has an aluminum base not (AC Delco plastic) OEM junk. The truck runs good but now I'm getting a PO340 code, which is crankshaft position sensor. My engine needs to be cleaned/degreased. Is it necessary to replace with a new sensor or could oil, dirt, etc be shorting it out. This is my second 4.3L Vortec V6. The first didn't have these problems and I drove it 205K miles before selling it. This engine has 160,000 miles and has been a total POS! I miss my 74 Ford F-100. No emissions garbage or money pit. Just regular maintenance and drive on. Have a great weekend. Thanks for your help!

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Thursday, May 20th, 2021 AT 7:27 PM

JACOBANDNICKOLAS

MECHANIC
109,644 POSTS

If there is dirt causing the signal between the sensor and the tone ring to fail or be weak, it could cause the problem. Here is a link that explains the most common symptoms of a bad crankshaft position sensor. However, a P0340 is a camshaft position sensor code. I believe the cam sensor on this vehicle is in the distributor. See pic below. Is the distributor new? Did you swap parts from the old to the new one?

https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor

If you find it does need to be replaced, here is a link that explains how it's done:

https://www.2carpros.com/articles/crankshaft-angle-sensor-replacement

Let me know what you find or if you have other questions.

Take care,
Joe

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Thursday, May 20th, 2021 AT 8:01 PM

STEVE BLACK

MEMBER
3 POSTS

I forget to plug the receptacle to back of distributor. Now I'm getting a PO134 Code Bank 1 Sensor 1 No activity detected. Checked the connection and it's soiled. Both upstream oxygen sensors are new.

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Sunday, May 23rd, 2021 AT 2:55 PM

JACOBANDNICKOLAS

MECHANIC
109,644 POSTS

When you say the connections are dirty, are they corroded, damaged, or broken? Also, inspect the wiring to the sensor. Because of its location, the wire and insulation on the wire could be damaged.

I attached the information about the code and the diagnostics below. Take a look through them and hopefully, it will help.

Let me know.

Joe

See pics below.

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