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302 Mid range miss

 
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348tripower
Baron


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PostPosted: Sun, Jun 07 2015, 5:56 am    Post subject: 302 Mid range miss Reply with quote

A friend has a Cobra kit car with a 1988 302. It used to run just fine. He has been chasing a mid range miss for weeks. It has a butchered harness but the ECC plug was still intact. I ran the codes and found the EGR to be by passed but nothing else out of the normal. He has changed the distributor, plugs and wires, Iac, tps and Maf sensor. Any ideas?
Thanks,
Don

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kb426
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PostPosted: Sun, Jun 07 2015, 7:11 am    Post subject: Reply with quote

Check the fuel pressure. Should be around 36 at idle and 40 with the vacuum line removed or above idle. If the fuel rails are original , there will be a shrader valve on them to make it easy.
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wayne petty
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PostPosted: Sun, Jun 07 2015, 11:35 pm    Post subject: Reply with quote

first thing.. has somebody thrown a new ignition coil on it.. they tend to leak high voltage out of the core to the inside of the laminations.. this is a gm coil ford will have a similar issue.. http://i.imgur.com/RXwLbcc.jpg

leaks will steal the highest voltages needed to jump the gap at the rotor to cap terminals and at the spark plugs.. creating a misfire..

edit.. next step.. print and perform this test.. http://i.imgur.com/WMDprhm.jpg

why.. if the braided ground strap from the back of the motor to the firewall is loose.. disconnected missing.. where ever the computer is stashed it won't have the same voltage on the ground side the engine does.. and it will cause a bunch of issues.. the voltage drop test has found issues for me so many times i do it on every car when i open the hood and before i close the hood.. it will find issues at the electron level.. its not the last diagnostic step. its the first so you don't spend hours or days chasing non issues .. print it and stick it to the wall so you can learn it. it really points out issues on new cars.. they really have issues on both the positive and negative sides..


there is an issue with ford EFI systems.. they really don't like to run without an functional EGR.. off idle to mid range when the system drops into open loop. the computer expects to have EGR flow reducing the percentage of oxygen in the incoming air charge. during open loop and even closed loop the ecm reduces the fuel injector pulse width as there is less fuel needed with the EGR flow reducing the oxygen content..

you will also want to reroute the drivers side spark plug wires.. i don't know if you have a straight EFI motor or the EFI HO motor. they have different firing orders.. they also have different ways of firing the injectors.

if one is using a HO motor in with a NON HO harness and computer it will have an odd misfire .. a non HO motor with a HO harness and computer makes it ever worse..

lets start simple... drivers side spark plug wire loom as the wires cross over the drivers side valve cover.. the wires need to be swapped in the holders. but NOT at the spark plugs move the wires to this order in the holder.. 7, 5, 6, 8. ford motors have the same issue that chevy motors have on some of the firing orders the last two cylinders fire 90 crank degrees apart..

on a chevy the 5 and 7 spark plug wires are short and still need to be separated or an inductive spark when the #7 spark comes down the wire the #8 picks up an inductive spark 90 degrees early.. that is with both valves closed.. and enough compression built up to cause combustion to part of the air and fuel in that chamber..when the piston gets to the top and the spark plug tries to light the mixture it will be Really lean. this will be like having a cutting torch in the cylinder..

there is another thing... unplug the oxygen sensor and see if the misfire goes away or changes.. these engines also had a thermactor system. also known as an air injection system..

the smog pump as the TAB valve.. thermactor air bypass.. this is controlled by one of the 3 relays on the top of the motor. . this allows the smog pump output to be dumped to the atmosphere. or directed into the hoses and tube to the diverter valve.. TAD.. thermactor air diverter. this is either on the side of the motor on a bracket or behind the intake..

the TAD valve controls where the air injection air is routed to.

when vacuum is applied to the TAD Valve. the air is send to the tube going to the side of the cat or to the pipe directly in front of the cat.. down stream of the oxygen sensors.. when NO vacuum is applied to the TAD valve the air injection output is directed to the cross over tube and either to passages in the back of the head or to the exhaust manifolds either of which will distribute it into the exhaust ports to assist burning of the excess hydrocarbons.. this is only done OPEN loop.. if it happens in closed loop.. you have serious issues with the oxygen sensors seeing too much oxygen in the exhaust stream this forces the air fuel ratio way crazy.

maybe i should have just posted this..

http://i.imgur.com/gdnABcJ.jpg

http://i.imgur.com/Q533dSC.jpg

http://i.imgur.com/56SsswX.jpg

http://i.imgur.com/Ymrag2D.jpg

i usually test this stuff with a vacuum gauge with a Vacuum TEE and a short piece of hose.. this i hook right into the system being tested.. the EGR will only flow test with the wheels spinning. and the Vehicle speed sensor hooked up to the computer properly.
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348tripower
Baron


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PostPosted: Wed, Jun 10 2015, 4:54 am    Post subject: Reply with quote

kb,
The fuel pressure is fine.
Wayne,
I passed this info along. He is beginning the checking and testing.

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