Engine cuts out
 

I'm looking for some trouble shooting tips. My Shell Valley has a box stock 87-93 FI 302 that feels like it runs out of air under full throttle starting around 2500 RPM. It feels like someone clamps a hand over the intake or shuts off the gas momentarily. Since my troubles usually occur from the last thing I tinkered with - the only thing I've done is remove and reinstall the upper plenum in order to install new valve covers. I thought I might try a run without the K&N aircleaner attached but would welcome any suggestions from someone who may have had a similar problem.

Regards,
Robin

Are we to believe this problem just cropped up since you worked on the engine?

What exactly did you change?

No - the problem has been gradually getting worse and did not manifest itself immediately after my tinkering.

May I suggest you check the air cleaner for size.

If it is small, it may not flow enough air for the engine requirements.

Does it fall on it's face when you jump on it?

What is the timing set at? Does it have a distributor?

Have you connected a fuel pressure guage to the fuel rail?

Just a few thoguhts

trularin,

Thanks for the suggestions. Yes teh car does as you descirbed - accelerates well then suddenly falls on its face as if my foot slipped off the accelerator. I would call the air cleaner small - about 8 inches in diameter. I'll try removing same to see if that is the problem. The car does have a distributor and I'll have to check the timing. It doesn't have a fuel pressure gage (yet). Will such a gage tell me what I need to know about fuel pressure parked or will the car need to be a full throttle on a dyno?

Thanks again for responding. It's been 30 years since I tinkered with cars.

Robin

Okay, the 8" cleaner probably will not flow the air you need.

I copied this formula from K&N:
Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them since very little air flows through this area.

A= (CID x RPM)/20839


A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute at maximum power

Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.

A= ( 350 x 5500 )/20839 = 92.4 Square Inches

If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) times its length. If you are sizing a round filter, use the following formula to determine the height of the filter.

H= (A/(d * 3.14159) ) + .75


A = effective filtering area
H = height
D = outside diameter of the filter
3.14 = pi
0.75 = the rubber end caps

I think you will find the air cleaner may only be good for 2000 to 2500.

Check your timing. I am not sure about your FI 302, but most engines that are built have a 32 to 38 degree total advance and an initial that will give them that. You may want to search "timing" on the site for more info.

If you have a fuel pressure guage, you can see if the rail gets depleted as you step on it. Say at idle you are running 35 to 40 pounds to the injectors. You nail it and as it climbs in RPM the fule pressure drops to 10 pounds. You will have miss fires and a complete loss of power.

Well, GTG. Keep us posted on what you find. Hope this helps.

This is outstanding information - thanks and I will get back to you.
Best wishes

I think I found the problem - at least part of it. The motor runs out of fuel under acceleration with less than a 1/3 tank full. As far as I can tell the pick up point is near the front of the tank. After a fill up the car pulled well to red line at least in 1st and second - need some freeway time to try third. I did the math and the air cleaner is too small - but removing it didn't make a difference - filling the tank did. I still plan on checking the timing - and installing a fuel pressure gage. Can't hurt to become more familiar with the motor. Thanks again for the helpful suggestions.

Regards,

Robin

Robin,
You should consider installing an aluminum fuel cell with a sump at the bottom with the fuel supply in the sump. That way, you can get good fuel flow even with just a gallon of fuel in it. Works great on mine.

Check the fuel filter its cheap to change.

The tank may have a filter on the pick up in the tank. If becoming plugged then the additional weight of fuel with a full tank helps push through occluded filter. You could also have a cracked /broken hose in tank that is exposed when fuel level is lower in tank. Bet you get to drop the tank to fix this one.

my car does the same thing with a 1/4 or less tank of fuel. over 1/2 a tank and no problem. fuel sloshes away from my tank fuel line pick up on hard acceleration when tank level is lower, the fuel does not get sucked into the line. i have a fuel pressure gauge , but in the engine bay, so who knows what it reads when i am driving, but with the hood up parked, i know what it is. LOL..so, not much help in real world of driving . looks good though, just not practical for anything but peeking at it when parked. i had a fuel pressure gauge mounted under the dash for a while but got an unpleasant thought about raw gas getting into the cockpit should something fail, like the pressure line or a fitting. KABOOM might be the result, so yanked it out of there . if you think air filter size is an issue, as was stated, pull the aircleaner off and see what happens for comparison. bill.

I use a temparay one SnapOff used to sell years ago. It allowed you to monitor fuel pressure through the windshield.

Once you determined the pump and delivery were good, you removed the guage and went to the next item to be checked.

I had the same problem with fuel starvation on my FI BMW , and burnt out two fuel pumps as a result. The Fi pumps are great at pushing fuel but terrible at sucking fuel so any fuel surge leads to drop in fuel pressure at the injection rail.

To solve the problem I put the swirl pot and integral pump out of the fuel tank of the donor car into the fuel tank in the Cobra, carefully positioned the return feed so that it fed retuned fuel into said swirl pot and the result has been a complete cure of the problem.

Now I can run the tank almost dry and theres still not a trace of hesitation or power loss. Try to emulate the donor cars fuel system as closely as possible and you shouldn't have a problem.

Cheers,

Tony:3DSMILE: