Engine decelerating and backfiring

BOWENFARMS

MEMBER
1993 CHEVROLET 1500
5.7L
V8
4WD
AUTOMATIC
170,000 MILES

I've got a 5.7L TBI-350, I'm having problems with it decelerating the engine and backfiring after I drive it 10-12 minutes or so. It will run fine then all of a sudden it looses power and drops to 500 rpm then after 5-15 seconds it goes back to normal. I've replaced the distributor cap and rotor still the same results. Any thoughts?

3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Hi,

Do you have a scan tool that you are able to monitor the o2 sensor data?

I am wondering if you have an o2 sensor that is causing an issue after the engine warms up. When the engine is cold (before it reaches operating temp) it is in open loop. This means it is not using o2 sensor data and others. So if once it is warmed up and it starts looking at that data and it runs poor then it may be supplying bad data.

Here is a video that shows corrections for backfiring. Clearly, some of this is not going to apply to your truck but it will get you thinking of possible causes. Let me know if you can look at o2 sensor data and we can go from there. Thanks

https://youtu.be/HiI_eeFg-2A

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Friday, August 30th, 2019 AT 1:55 PM

BOWENFARMS

MEMBER
2 POSTS

KaseKenny1, 4 hours after posting that the truck worsened. Now it will run for a little bit then sputter and lose power and shut off. Usually acted up worse over 1,000 rpm's, idled okay but you could only go 3/4 of a mile and it would sputter. I tried to make it home with the truck and after fighting it for 10 miles it shut off and smelt like unburnt gasoline and kinda smelt like smoke a little (but may not have been). Anyone know something it could be?

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Saturday, August 31st, 2019 AT 6:36 PM

KASEKENNY

MECHANIC
18,907 POSTS

It could be a number of things but we are going to have to do some testing to limit it down otherwise, we would just be guessing at sensors and parts to replace. If there were one thing of any other, I would tell you to just try it but unfortunately, this could be as simple as a sensor or it could be a valve-train issue.

Let's start with any codes. If you don't have a scan tool then a parts store can pull them for you free of charge.

Then we need to check the compression on each cylinder. If we don't have a solid engine with good compression then all else doesn't matter.

The raw fuel smell is clearly signs of a misfire but we need to start to eliminating things.

Here is a guide that will help in case you are not sure how to perform this test:

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

Let me know if you have questions and we can go from there. Thanks

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Monday, September 2nd, 2019 AT 2:54 PM