Won't idle/leaning out

[Alisha]

Hi, I'm hoping someone here can help....

We have a 347 with fuel injection. We've always had problems with the car leaning out. We had a chip made for the computer (was told in the beginning our computer would adjust to the engine...it didn't) which seemed to help but according to the guy who built our engine, it's still not enough...the car still needs more gas. We were using 36 lb injectors with a 70mm mass air meter (maybe 75...can't remember but it did match the injector size) ...he suggested we go up to 42 lb injectors to get more fuel which would also mean we needed an 81 mm mass air meter. We went to order both parts, we explained what the car was doing and was told it wouldn't make sense to change the injectors and the mass air meter. Said if we needed more gas and less air then we shouldn't change the mass air meter...or to at least try the larger injectors first.

We've installed the larger injectors and it is running much more rich. Now it won't idle correctly at all...

That's another problem we've had as well from the beginning...the car "surges" (I think that's what it was called). Before the larger injectors the car wouldn't idle correctly when in drive but if in park it was fine. The idle would jump up and down when in drive...like it didn't know where it should be causing the car to rock back and forth. Now with the larger injectors it's much worse. When first started it idles at 1000 but after about a minute it drops to 500 then jumps to around 1300+. It'll jump up and down 5-6 times then die. When in drive it won't hold an idle at all...you have to constantly give it gas to keep it running.

Any ideas?

[Tom Wells]

Hi Alisha,

Please regard this long-distance diagnosis as worth what you paid for it

It sounds to me as if you went the opposite way from what you needed to go.

Smaller injectors may help your problem.

Larger injectors can cause the no-idle problem you describe. The computer and injectors can support only so short a time for the injector to be open. If the O2 sensor keeps telling the computer the engine is too rich at idle, the computer keeps trying to cut back the time the injector is open. The larger the injector, the bigger the shot of gas at any given timing. Eventually the computer practically has to shut them off

My 521 CI engine works fine with 36 pound injectors, producing 412 HP at the rear tires through a power-robbing C6 automatic trans, suggesting a lot more at the flywheel. The air/fuel ratio is fine from idle up to max rpm, suggesting the injectors are plenty large enough.

Unless you have a supercharger and are producing well over 500 HP, you most likely need smaller injectors, not larger; say 30 pounds.

Try going to a dyno facility if there's one nearby. You should be able to verify the engine is way too rich at idle.

Hope this helps,

Tom

[John Poling]

Alisha,

Where in Indiana are you located? As stated by Tom wells, the best bet would be to take it to a Dyno equipped shop that has the ability to work on it also and is competent enough to tune it. There is a place here in Fort Wayne that is so equipped with a Mustang Dynomometer that can read up to 1750 HP and they have some very knowledgeable people there. The company name is EPP - Exotic Performance Plus. Here is the web site http://www.exoticperformanceplus.com/. Check it out if you live close to Fort Wayne it's absolutely worth the trip.

John

[Alisha]

Thanks for the info!

I talked to the guy who built the engine a little while ago. He's not much on fuel injection (has tried to get us to change more than once) but said he came to the conclusion of needing bigger injectors because each time he increased the fuel presure with the 36lb injectors it helped...he had the fuel presure maxed to what the lines can hold but the engine needed more. So the only other thing he could think of was bigger injectors.

Tom, what you're saying makes sense...I'm gonna run it by him and see what he says. We're willing to try just about anything at this point!

John, we're about 4 hrs from Fort Wayne...would be money/time well spent I'm sure.

[HighPlainsDrifter]

Hi,
I'm assuming you have a factory EFI eecIV setup.
You can do a couple things to help.
1- get a digital voltmeter and set the TPS sensor on the throttle body to0.991 volts.
2- set the fuel pressure to 40 psi [engine not running]
3- drill a small 3/32" hole in the throttle air plate. This allows more air in the engine at idle,the computer is opening and closing the bypass valve on the side of the throttle body causing alot of up and down rpm's.
4- set your timing at 12 BTDC[ with the black shorting pin out]

Good Luck,
Perry.

[Dwight]

Alisha,

DID YOU GET YOUR 347 STRAIGHTEN OUT? I KNOW IT WAS 2004 WHEN YOU POSTED BUT DID YOU FIND A SOLUTION?
DWIGHT