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Charter Member
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348 Posts
Discussion Starter · #1 ·
Build is 90GT 302, E303 cam, GT40P heads, 19Lb injectors (standard EFI set up). This is what happens -

On start up it is like having a 'digital engine' it's either going to be normal or to all intents has a miss/timing type of issue. It will still drive when 'not right' but you can feel the engine kind of fighting to get back in tune. Then at medium revs (around 3K) it will on occasions just fix itself and run normal, once again like a switch being flicked.

It is as though a sensor/connection is bad but when checked all the connections and readings (via DMM) are good.

No codes reported.

Any ideas would be welcome, all the basic checks done, timing, fuel pressure (39 PSI), firing order all good.

Only clue I have is when first starting the engine after rebuild the timing was very erratic and was fluctuating badly. This was at the time I had issues with the TFI module but seemed to settle down when new TFI fitted.

Then I noticed this 'digital' effect.

Cheers, John
 

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Not a waxer
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12,092 Posts
Shot in the dark but I've seen it work before with intermitant mystery ailments; start by cleaning all of the pins at the "salt & pepper" connectors. Afterwards take a small pick and spread the male pins slightly to assure good engagement.

Jeff
 

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Charter Member
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348 Posts
Discussion Starter · #3 ·
Thanks Jeff, but during the wiring of the car I deleted the 'salt and pepper' connectors by hard-wiring so I can rule that one out :)

Cheer, John
 

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Junior Charter Member
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2,472 Posts
If you or a friend has a spare distributor. Put it in and see if that fixes it. PIP, TGE or worn bushing might be an issue.
I assume the carbon in the top of the cap is fine and the rotor is good.
 

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Blue Oval Scribe
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9,175 Posts
Verify all engine and ECM grounds...

Ensure the block is grounded to the chassis, your ECM has a solid ground, etc. If need be, try running a ground wire for the ECM directly to the battery as a test.

HTH...
Mark
 

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Charter Member
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348 Posts
Discussion Starter · #6 ·
Thanks guys, I'll follow up on suggestions.

Steve - distributor is brand new, about 1000 miles only ran in total so innards should all be OK from a wear perspective.

Cheers, John
 

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Registered
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293 Posts
I called my mechanic , he is old school and fixes cars that others give up . He said the Engine coolant temperature sensor ,The one that tells the computer the engine temp , not the one for the gauge on the dash, he said it tells how much fuel/air to send and receive . He mentioned also that cold plugs will give that problem and the easiest is swap the computer, I was gonna say to put a heavy ground on the block Good Luck
 

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Official OLD GUY
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3,877 Posts
First choice

Engine coolant temperature sensor . . .

This is the sensor that tells the engine to hold a 1200 - 1500 RPM when first started. If your engine doesn't do that, that's where I'd start looking.
If this sensor is bad (or going bad), it will not tell the EEC to enrichen the fuel and retard the spark when cold at first startup. Also, check for condensation under your distributor cap. Cold mornings will cause condensation to accumulate there and affect the first few minutes of running until it evaporates. Verify all your grounds . . . especially your engine to chassis grounds. Many things that require a ground depend on the path thru the engine block to chassis. If they're bad, you'll get intermittant conditions for a number of things.

HTH

Doc :beerchug:
 

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Charter Member
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348 Posts
Discussion Starter · #11 ·
Thanks all, great input, I'll check all over Xmas and get back with you.

Cheers, merry Xmas, John
 

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FFCobra Craftsman
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25,961 Posts
When you check grounds don't just use an ohm meter. The current the meter uses for continuity/resistence is different from that used by sensors so the results don't always work out.Take every ground apart,clean all paint,rust etc from the surface down to bare metal. Put it together and coat it w/ grease to stop rust. Not currently haveing an EFI engine I don't remember if there are sensors in the intake. If there are, run a small ground wire from intake to head. Run another from head to block. And check the block to chassis ground as above. Over the years I have seen many electrical components including an electrically controlled transaxle changed to no effect. The a loos or bad ground was found to be the problem.
Another path to take is checking for vacuum leaks. There are a couple that come from under the intake and real hard to get to. New hoses are not a bad idea. Also check the bellows boot from the throttle body to the AFM. Don't just look at it. Take it off and bend it around looking for cracks in the valleys of the bellows. these open and close as the engine moves on it's mounts and are near impossible to see when on the car.HTH.
 
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