Intake Backfire

[daepp]

The facts:

Newly rebuilt 2.4T, JE Pistons @ 9.0-9.5 CR, ported T heads, 91 Octane, E Cams, T MFI Pump and Throttle bodies w/57K miles useage.


The Story:
Runs very strong and rich, with lots of HP for a little 72. However, at idle I get what I believe is an intake backfire/click sound once the engine is warm. Will this hurt my engine's longevity? Can it be eliminated? Due to the car's low mileage I do not suspect excessive wear in the TB's.

The solution:

?

[daepp]

Bump

Anyone?

Bueller?

[Va914]

I have the same issue with my Webers.. I have read over and over agin it is mostly caused by a lean condition due to air leaks or miss jetting or?

[daepp]

Has anyone ever suggested that it can damage the engine?

And what about changing the timing?

[Superman]

Intake popping is usually a sign of a lean running condition. But ignore that and listen to this: Every reliable bit aof advice I have seen here on this subject suggests that you carefully follow the MFI tuning procedures. Measure, Check, Adjust.....or something like that.

[David E. Clark]

"Popping" is usually caused by a too lean condition but it can also be caused by a too rich condition. Since you stated that your car was "very strong and rich" I'd look into the mixture being too rich. What the popping symptom is telling you is that the plugs are not firing correctly which is usually a mixture issue but could also possibly be an exhaust gasket (maybe even a head gasket) leak. I would work my way back through Mechanical Fuel Injection, Check Measure Adjust in exact detail to get your mixture correct. Also take a look at the links under my signature for lots of good MFI information.

P.S. If you put MFI in your subject line next time you'll probably get more responses. Us MFI guys like to stick together.

[daepp]

Superman and David (SuperMFI Guy) Clark: Thanks for the info.

With respect to CMA, is it true that it is only reliable for stock setups? Having changed such things as air/intake flow and CR, I suspected that the CMA would not "calibrate".

The pump has been leaned (?) out a bit to improve idle, but it's still quite rich. It's rich to the extent that it'll make your eyes water with the top off at a stop light! Of course, when you get on it, it goes like hell. It also idles pretty good. However, am I ruining my intake valves, seatsor guides?

[daepp]

Bump

[David E. Clark]

Uber,

I only have experience with the stock MFI system. Like you, I have read that MFI is really only applicable to the stock system. My impression is that the specific details may not apply if you have a modified system but that the general outline for checking the system is still applicable no matter what you have done to your MFI system. Even though you have a modified system, I would go back through my system in the order suggested (mandated):

1) Air cleaner cartridge
2) Compression loss
3) Spark plugs (spark plug connectors)
4) Dwell angle
5) Ignition timing
6) Fuel pressure and flow
7) Injection nozzles
8) Injection timing
9) Correlation
10) Exhaust emission test
a) at part load
b) at idle speed

No matter how you have modified the system, it still, for example, won't run correctly if you have a bad cylinder from compression loss. After going through the above sequence, and if you do determine that you have your MFI pump set too rich (and how can it not be if it makes your eyes water at a stop light!), remember the phrase "I Lean Left" for "idle screw lean to the left-- and then remember that the main rack is the opposite of the idle rack, so the main rack is lean right, rich left. Seems like I always need to remember this.

As far as to why your car seems so rich at idle but still runs great. Tyson Schmidt posted this explanation of why MFI cars run rich at idle but get leaner when they are driven hard in this thread.

Originally posted by Tyson Schmidt
MFI actually gets leaner under load, since it really has no way of measuring load other than by throttle position and rpm. That's why they run so well under acceleration when set very rich.

You have to set them rich while free-revving so that they are not too lean under load. So if they are set to be a little rich under load, they are really rich under all other conditions, especially during warm-up.

MFI engines will run very well with overly rich mixtures compared to carbs or EFI, since the injection pressure is around 250psi, resulting in excellent fuel atomization. And I suspect that they likely clean the plugs off more efficiently under acceleration due to the leaning out effect I mentioned earlier, so they keep running relatively well with too rich a mixture that carbs just wouldn't tolerate.

Then if you delete the fuel shut-off during over-run feature, you are dumping a lot of fuel during over-run. This is actually fine, so long as you don't do a lot of deceleration with your foot off the gas, such as compression braking in every gear, or down long steep grades.

If you delete the fuel shut-off, then you need to change your driving habits to using the brakes, not the engine to slow the car down.

Finally, as to the question of ruining your intake valves etc, I have to say that I don't believe that the "popping" by itself will have a major negative effect. This seems to me to be merely a symptom of a larger problem. The over-rich fuel mixture, on the other hand, can be very detrimental. What is basically happening with your mixture being that rich is that you are filling your oil tank with gas because such a rich mixture will always allow gas to run past the cylinder rings and into the oil tank. The most damaging part about diluting the oil with gasoline from the overly rich mixture is that the thinner gas/oil mixture gets consumed and burned off by the engine at a much more rapid rate than a complete oil mixture would. This creates more room in the oil tank for - you guessed it- more gasoline which gets burned off even more quickly and on and on. One hard run in this condition will cause the "oil" temperature to go way up and since gas evaporates and burns off in a fraction of the time it takes for oil to evaporate you will inevitably be left with insufficient lubrication for the engine.

It goes without saying that since a gas/oil mixture doesn't lubricate very well it will ruin your valves cams, rockers, chains, etc in no time.

My bottom line: If your eyes are watering at stop lights from breathing in gas fumes caused by rich running, you absolutely must get this fixed or you risk killing your engine from lack of proper lubrication. Hope this helps.

[daepp]

The depth of this BBS never cease to amaze me!

Thanks for the great info, David. Your suggestions look like a good 12 step program. I'll post the results once everything is checked.

David