J16

I have a 96 Impala with 77000miles . Super clean and cared for which I bought from older retired gentleman.

I have been chasing a intake back fire.
Here is video

You Tube

The intake back fire only happens off of idle when you hammer throttle fast. if you get into throttle slowly no issues.
First repair I completed was fuel system. This originally was low psi so I thought to start here. (it was around 20 psi)

-New fuel pump
-New sending unit
-New Delco fuel filter
-New Bosch injectors
-New Delco fuel pressure regulator.

You Tube

As you can see in video system pressure looking good . This really helped car cold start etc .glad I did this all.

I am now confident fuel system is now corrected.
Moving on

Tune up and optispark
-New Delco O2 sensors all 4.
-New Delco coolant temp sensor.
-New Delco MAF.
-New NGK spark plugs and Delco wires
-New Delco opti cap and rotor.
-New Delco PCV valve
I used a mini scope and from what others telling me its looking correct and clean signals. This test is at the opti spark connector.




This is testing at the ICM




​​​​​​
​​​​​​
Next testing

I used a smoke machine to check for vacuum leaks. No smoke no vac leaks detected.
I also ran sea foam into intake just to give her a cleaning.

Next test was to confirm exhaust system is not clogged. I did both sides. I removed the upstream 02 and reading came back at idle and around 3000rpm (0-0.5 psi )
so confirmed exhaust is not blocked.


Scanner has no trouble codes..................

Fuel trims
STFT B1 2%
STFT B2 2%
LTFT B1 -1%
LTFT B2 5%

my understanding is if the total stays under 10 we are ok on older cars

Next testing spark advance
at idle we are at 16.
at 2500 rpm 37.

Lastly I wanted to test knock system some more.
I noticed my knock count activity is in the red which has me wondering why (see vid)

You Tube

Testing I have performed to KS.
Back probed KS and getting 4.87v and with wire unplugged from KS 4.97 so that's looking good. This is with key on not running.

With car running around 4.87v.
Next up connecting scope. Looking good and with rev up clean signal.(see pic)
I also removed both KS from car and fit onto vise. My ohm reading were in spec.
Spec from GM 93K to 107K. I was around 98k so both are in spec.

I then connected to scope while on the vise and used hammer to create signal and they are beautiful on the scope.
I will work on loading more in images but this is where I am at

Thank you I hope to one day enjoy this car. if not I can film car at junk yard getting crushed

atlantadan

Look at what sort of data your throttle position sensor is sending. I love your oscilloscope. Which one is it?

SS RRR

iTrader: (2)

That sounds like a classic case of plug wires being on the wrong plugs. Double/triple/quadruple check to ensure they are routed correctly. Backfiring almost always leads to either a vacuum or timing issue.

SS RRR

iTrader: (2)

Check them again. Doesn't matter if intake or exhaust backfire when it's a timing issue. Combustion can come out of either valve. As stated a vacuum leak can also cause an intake backfire. Engine is not accelerating like it should when you blip the throttle, which is leading me to those two possible causes.

J16

I will check this week again on wires I wish it was something this easy but who knows. I will report back soon.

As to vacuum leak i did perform a smoke machine test and found no leaks not sure what ease i can do.

and to confirm this is what it should be

BALLSS

Alex, I will take EGR for $100

stuck EGR can cause stumble on tip in.

SS RRR

iTrader: (2)

possible it's an EGR failure. Usually those don't act up until there's a high amount of vacuum put on them at cruising speeds. Did you check the EGR gasket surface with the smoke machine? Can also spray starter fluid/brake clean/carb cleaner to see if the idle changes.

J16

Today I was able to take a video to show you . What do you think?

​​​​​​

J16

I checked once again the installation of ignition wires they are correct. I followed each one back to Opti.

pdxmotorhead

I've had a cracked charcoal canister cause some truly weird events.

J16

When I set up smoke machine I tapped into the intake manifold and let machine run a good 5-10 min to really fill it up. I looked around all over for smoke didn't see anything. I tried this 2 times 2 different days.

Is there more testing I can do on EGR or should I just replace it?

Fast355

Scope the MAP and TPS. Both are responsible for fueling enrichment especially on throttle stomp. A momentary lean spike can cause an intake backfire.

JimCT_9C1

Here is my approach to testing EGR system:

1. Engine Off - Check the valve can be moved manually with finger or tool (valve not frozen)

2. Engine Off - Test valve diaphragm by disconnecting vacuum line at EGR valve. Move valve by hand and place finger over vacuum port, or apply vacuum with pump (valve should hold position and vacuum, should move then hold position and vacuum using pump)

3. Engine Idling - Move the valve manually or with vacuum pump while idling to see if you get a stumble (yes is good - internal EGR passages open)

4. Drive Test - Disconnect vacuum line at EGR valve, attach vacuum gage and place where you can see it while driving. At light/med cruise should see the gage move from zero to near manifold vacuum. May have variable vacuum since this is not just an on-off solenoid but is rapidly pulsed by PCM. Will drop back to zero under other conditions. Scan tool is great here to see the PCM commands and the vacuum gage response. (problems here could be the solenoid valve, wiring, vacuum lines,).

5. Continue with basic vacuum, mechanical, electrical checks as needed to isolate problems in those areas.

Hope this helps -

Jim

ACE1252

With a dual channel scope, you may be able to scope the MAP and Low res opti signal to see if the same cylinder is causing the back fire all the time. A cranking relative compression test using an amp clamp might also give you an idea if there is a compression/valve lash/wear issue. If it shows a compression issue, follow it up with the standard compression testing. Leak down testing on each cylinder might not be a bad idea either.

SS RRR

iTrader: (2)

Here's a shade tree way to test if you have an EGR leak- Either buy or make a block off plate using a piece of aluminum and cut it to size/drill bolt holes using an EGR gasket. Bolt into place with gasket. See what happens. Keep EGR plugged into harness so the SES light doesn't trigger.
Has this backfire only been in the intake or has there been any through the exhaust?

SAPPER

iTrader: (10)

Leakdown test????
Through the intake could be a valve / valve train issue

J16

My understanding is that they use 5v, ground and a signal wire. I will scope signal wire to see how they look and report video soon. Just gotta find wiring diagram thank you.


It does help very much thank you I will try and report back

Interesting thank you. Good information. The scope I am using is only 1 channel but I do have OTC low and med amp clamps plus I also have compression tester and leak down tester but trying to hold off doing that its a pain to remove spark plugs again.

Yes since day one its been only intake just like video low rpm you step on it and it pops high rpm no issues

lets hope its not other wise you guys will be seeing a video of me scrapping this car lol

pdxmotorhead

Just noticed you have the K&N, have you looked to see if the MAF is clean and still has the honey comb air guide in it?
Mafs do not like turbulence. That setup just seems like it would work better with the MAF half way between the Filter and throttle body..
Both MAF cars I owned need some tlc on the maf regularly..

J16

the maf is brand new delco part it has screen just like oem .

that filter set up is a kit made for impala not custom

pdxmotorhead

AH that makes sense, any way you could get 4" of tube between the MAF and filter?
(It looks like the filter is bolted straight to the MAF in one of the pics? I may be not seeing it well.
I worked on a Camaro that had a random stutter/pop that turned out went away
when we got some tube ahead of the MAF..