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351w Performance Build- Fuel Delivery Issue

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Old 07-09-2018, 02:30 PM
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351w Performance Build- Fuel Delivery Issue

I'm having a little bit of an issue with my engine running lean. Last year I swapped out the 302 for a 351. It's mildly built- GT40P heads, LT headers, stock roller cam (F4TE), Edelbrock Truck intake. Under a mid to heavy load, it will ping. If I put premium gasoline (here at 5000' we have 91 octane) in it it won't ping as bad, but is still there. Does this sound like a fuel delivery issue because the performance mods are too much for the stock fuel system, or something else is going on? It's been doing this ever since I did the swap last summer, 12,000 miles ago. Would I need an adjustable fuel regulator and/or a high volume fuel pumps to correct this? There is a small oil leak coming from the lower intake gasket, I think. Is it possible this is causing unmetered air to enter into the combustion chamber? I am using the MAF that was originally equipped for this truck. Any and all help is greatly appreciated!
 
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Old 07-09-2018, 03:02 PM
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where do you have your timing at...
 
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Old 07-09-2018, 03:06 PM
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I forgot to mention that. Timing is set at 10* BTDC. Also, using stock 19# injectors, 302 ECM and wiring harness. Plugs/wires, distributor cap/rotor all new.
 
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Old 07-09-2018, 03:45 PM
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Did you happen to reuse the 302 injectors and if so did you clean then? May have some clogged injectors causing a lean issue. Also, should use a positive stop to make sure the tdc 0 degree mark is still where its supposed to be. I've seen them a few to several degrees off, you may be seeing 10 degrees and it could actually be 14 or 16.
 
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Old 07-09-2018, 03:51 PM
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MAF or Speed density?
 
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Old 07-09-2018, 03:56 PM
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I used "refurbished" injectors from a seller off eBay. They all appeared to be good when I installed them, however, I did reuse one of the injectors that came off the junkyard 351 when one got clogged (#8) because I didn't clean out the 351 fuel rail well enough. I'll check that injector, maybe that's where the problem lies.

The balancer I used was new, but like you said, it's possible it shifted and doesn't show an accurate reading. How do I go about using a positive stop to check for this?

Thanks very much!
 
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Old 07-09-2018, 03:57 PM
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Originally Posted by houtxfdcapt View Post
MAF or Speed density?
MAF from the 302.
 
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Old 07-09-2018, 04:14 PM
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In theory even though its a 302 maf, it should signal the appropriate amount of fuel, for air measured for the 351. I too would lean towards checking your injectors.. OBD I or OBD II? O2 sensors in place?
 
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Old 07-09-2018, 04:21 PM
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Right, I do recall prior to swapping in the 351 with these performance mods, the general consensus was the 302 MAF would run the 351, but possible this combination would exceed the limits of the stock fuel system. It's OBD 1 and the stock (new) O2 sensor is in place.

I didn't pull all 8 plugs, but the few that I did were ashy/white. If I had a partially clogged injector/s, wouldn't only those be affected?
 
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Old 07-09-2018, 04:42 PM
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Originally Posted by EllieMae94 View Post
I didn't pull all 8 plugs, but the few that I did were ashy/white.
That means the motor is running lean overall, have you checked fuel pressure? One thing to try would be to disconnect the O2 sensor to force open loop and see if that makes any difference. If it does then something is making the computer pull too much fuel out, it could be the O2 but it could also be the MAF meter or there could be an unmetered air leak somewhere causing it.

 
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Old 07-09-2018, 04:52 PM
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I last checked fuel pressure a few months ago. I don't remember the readings now, but it was within spec, nor did it bleed down after 30 mins after shut off. I was having the same issue then, so it isn't a new problem. I'll disconnect the O2 sensor as you say and will see if that makes any difference.
 
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Old 07-09-2018, 04:55 PM
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Originally Posted by EllieMae94 View Post

The balancer I used was new, but like you said, it's possible it shifted and doesn't show an accurate reading. How do I go about using a positive stop to check for this?

Thanks very much!
Buy one of these, or smash out the porcelain of a spark plug and weld a bolt into it. They thread into the spark plug hole, then you rotate the engine by hand clockwise til it hits the stop and mark the balancer at the tab. Then by hand counterclockwise til it hits the stop, mark the balancer at the tab. And half way between the two is real TDC.



 
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Old 07-09-2018, 05:02 PM
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Mudsports on the right track... running a 302 firing order or 351? and you can do a smoke test for air leaks in the system post maf or aftermath
 
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Old 07-09-2018, 05:06 PM
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The firing order is the same for the 302 that came out of it (1994) and 351 (1996) that went in. I was unaware of that piston stop tool, I'll be sure to pick one up. Also, I can check for air leaks using the method you describe above once I disconnect the O2 sensor that Paul suggested. Thanks to everyone for all their help!
 
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Old 07-09-2018, 05:08 PM
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oh and there is a thread somewhere about not being able to swap a 302 throttle body with the 351 throttle body.. but not sure that would cause your lean issue, think it dealt with proper limits of the IAC between them
 
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