97 yukon backfireing thru intake.

[nokuy97]

Hi,
I have a 97 with a 5.7 with 145,000 on the clock. I let a friend use it for the last couple of months, next it gets droped off in my yard by a flat bed. He tells me it's been running fine until he went out one morning and it just cranked and almost started and then he killed the battery.
I went out and checked the fuel pressure and when I turn the key on the gauge goes up to 60psi , so then I am thinking spark problem. I took off the dis. cap and rotor and they don't look bad and put them back. Then I checked for codes and there are none. SOOO what do you think , the coil maybe. If you let it sit a day and try to start it wants to but it just acts like it is flooded and keeps back fireing out the intake ?

Any ideas ?

[retorq]

Back firing thru the intake is a timing issue ... check base timing, verify TDC and that the distributor is actually turning when the engine is cranked ...

[bowtiefreak]

^^^timing

[black95]

+1 timing

[nokuy97]

So in other words it may have jumped time in some way. I will check that the dis. is turning and make sure the timing is correct.


Thanks

[SunlitComet]

Try this:


DEFINITION
The fuel ignites in the intake manifold, or in the exhaust system, making a loud popping noise.

PRELIMINARY CHECK
Refer to Important Preliminary Checks . See: Computers and Control Systems\Testing and Inspection

IGNITION SYSTEM CHECKS

* Check for the proper ignition coil output voltage with the spark tester J 26792 or the equivalent.
* Check the spark plugs.
* Remove the spark plugs.
* Check for the following items:
* Wet plugs
* Cracks
* Wear
* Improper gap
* Burned electrodes
* Heavy deposits
* Check the ignition system. Refer to Enhanced Ignition System . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\Enhanced Ignition System Check
* Check for crossfire between spark plugs (distributor cap, spark plug wires, and proper routing of the plug wires). Refer to Engine Electrical.

ENGINE MECHANICAL CHECKS

* Check the engine for the following the items:
* Engine compression
* Improper valve timing
* Intake manifold gaskets, refer to Engine Mechanical .
* Sticking or leaking valves, refer to Engine Mechanical .
* Check the EGR operation for being open all the time. Refer to EGR System Diagnosis . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\EGR System Check
* Check the intake and exhaust system for casting flash or other restrictions. Refer to Engine Mechanical .

FUEL SYSTEM CHECKS

* Perform a fuel system diagnosis. Refer to Fuel System Diagnosis . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\Fuel System Diagnosis
* Perform a fuel injector coil test. Refer to Fuel Injector Coil Test . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\Fuel Injector Coil Test
* Perform a fuel injector balance test. Refer to Fuel Injector Balance Test . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\Fuel Injector Balance Test Procedure

ADDITIONAL CHECKs

* Check the EGR gasket for a faulty or loose fit.
* Check the EGR Valve operation for being open all the time. Refer to EGR System Diagnosis . See: Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics\EGR System Check

[99Yuk]

My bet is on sheered Distributor Gear Teeth. I know, I just went through this before the new motor. Same symptoms. And I know how it happenes too. Buddy was driving it like it wasn't his. Tromping it at every light. That's how mine sheared. Stomped on it, and got up to 80kph, then nothing, it just died on the spot. Same flooding, backfiring as you. Needless to say, I don't do that any more on my new motor. Just saying, it's probably the dizzy gear teeth, which means buddy was abusing your rig.

[SunlitComet]

I want to check mine but I am not taking anything apart until I have my diag interface to realign the timing and get some live data feedback. I am tired of other mechanics giving me different explanations as what codes show up.

[ezdaar]

It's simple, it jumped time .
On a 145k 5.7, first thing I would do is pu a timing light on it and watch
The timIng marks closely. If. The mark wobbles back and forth constantly it's your timing chain.

5.7's are notorious for wearing out the chain and gears and jumpin time.
Buy a dbl roller timing chain. They like 30-45$ and you caneSily replace it in like 2 hours.
A 14, 10 and 12 mm socket wrench, pry bar and 3 jAw gear puller.
I've done so many it's ridiculous. You will need waterpump and timing cover gaskets aswell.

[nokuy97]

Problem is that it won't run at all to check the timing.

---------- Post added at 03:56 PM ---------- Previous post was at 03:52 PM ----------

Would a weak coil cause the same problems , say the spark just ain't there to fire the fuel enough ?

[ezdaar]

Pull distributor as mentioned above, check the gear.
If its plastic immidiatly put a broze gear on it. regaurdless if its broke, worn , whatever.

Next pull water pump, harmonic balencer and timing cover.

take a look at the timing chain gears teeth. some will probly be chipped or broken completly.
more likely just extremly worn aka grooves in the face of the teeth.
along with ALOT of slack in the chain.

regardless of wear, replace it with this.
http://www.summitracing.com/parts/SUM-G6601/

------------------------------------------------

This lil item
http://www.autozone.com/autozone/parts/1997-Chevrolet-Tahoe-2WD/Ignition-Control-Module/_/N-j9305Z9n80t

Is notorious for going out at any time on older chevy / gmc's
If your not getting spark. my bet is its bad. you can replace it with distributor installed.
just pop cap and remove the 2 lil screws holding it down.
beshure to put the thermal greese on it.
I changed one in a parking lot at night with no light on a suburban one night. lol

I would change this last part first before anything else.
But once running, I would still replace the timing chain and gears. your vehical will run noticably smoother.

[99Yuk]

BTW, if you are going to check the distributor gear teeth, you should use a marker or something to mark where it was positioned before removing it. Just in case it is still good, you can align up the marks to get it back in the same tooth as before. :)

[ezdaar]

BTW, if you are going to check the distributor gear teeth, you should use a marker or something to mark where it was positioned before removing it. Just in case it is still good, you can align up the marks to get it back in the same tooth as before. :)

Ahh yea, LOL good call on that one.. completly forgot about marking the base to block.

[nokuy97]

The only problem I have is that it must have some spark to make it backfire .Can a coil get weak and still do it's job until it finnally craps out ?

[ezdaar]

yup.

Dude if it jumped time that would cause it to backfire Badly through the intake.
Check the timing chain.
I have had numerous chevy 305's and 350's jump time. See I used to have a habbit of blowing up motors due to over revving them and replacing them with high milage junkyard engines. Sooo.... I have had a few jump time.

[nokuy97]

Yea
the last time I had a timing chain jump was on my 1976 3/4 4x4 chevy and I changed the chain and gears then. I got to wait until it warms up here alittle because this is going to be a outside job.

[99Yuk]

Yea
the last time I had a timing chain jump was on my 1976 3/4 4x4 chevy and I changed the chain and gears then. I got to wait until it warms up here alittle because this is going to be a outside job.

HAHA... I just aquired a 1976 3/4ton 4x4 Chev last year. It's my snow plow truck after the guys here thought it would be a bad thing to install on my Yukon. http://www.tahoeyukonforum.com/forum/album.php?albumid=419
I love it! It's so much fun!

[nokuy97]

Great TRUCK !!! mine got to the point that when you started it after sitting for a week or two , it would use a half of a quart of oil. Great when you can clear out all the bugs for a half mile away in the woods , smoke like hell but ran like a bear.

[ravingmadman]

A weak coil won't cause a backfire like that, not to the extent you're suggesting. If the timimg is off, then the valves are open when the spark ignites the fuel/air mix, which is why you'll see flame pop out of the throttle body. That's really hard on your engine. If I checked anything first, it would be the distributor. But as mentioned, make sure you mark both the rotor to cap orientation, and the cap to base orientation, in the event you don't find anything. Happy hunting!! Oh, and your buddy absolutely owes you a case of beer.
:drunk: