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  #1  
Old 01-11-2015, 02:11 AM
qinbrsto qinbrsto is offline
 
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'86 Bronco 5.0 FI hot start mystery

Hey all!
I have a 1986 Bronco 5.0 FI 4 speed no O.D. and it has an impossible fault. When my truck is cold, she starts just fine. No hesitation in any way. However, once I've driven for a few minutes and it has an opportunity to warm up, it hardly turn over until it's cooled down again. When attempting to start it, it acts like the battery is dead. It will turn over incredibly slow, about a single revolution per second. Not nearly quick enough to fire. Sometimes if I'm lucky, it'll kinda 'catch' and fire right up. Not commonly, though. If I'm able to pop/bump start it, it'll fire right up. Jumping the battery does nothing.
This is obviously a pretty big problem, especially considering it takes between 0.5-2 hours to cool down enough to start again. It is a manual transmission, and I have 35" tires with the stock 3.55 gearing, so it isn't difficult to kill. I just can't trust it to drive.
I have tried many different approaches to fixing the problem. New starter, starter wire, starter solenoid, battery leads, battery, and alternator. Because it has extreme reluctance to actually turning the motor over, but no problem running after pop starting it, I'm sure it has nothing to do with the ignition. However, I've still installed new plugs, wires, cap, rotor, both fuel pumps, and ignition module. Nothing made any difference.
I thought it may be a pulley on the serpentine pattern possibly heating up and causing drag. I inspected all of the pulleys, and they all looked fine other than the A/C compressor. I took the V belt off of it, and tried again. Nothing.
I start it in neutral with the clutch in, but I've tried it with the clutch out as well. (Faulty clutch depression switch) Nothing makes a difference.

I am so sick of blindly replacing parts, and it's time to farm this out. I really hope someone can give me any other ideas, because I'm completely out.

Thanks in advance!!
/Quinn

Last edited by qinbrsto; 01-11-2015 at 02:14 AM.
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  #2  
Old 01-11-2015, 09:19 AM
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dustybumpers dustybumpers is offline
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Sounds like your timing is way advanced.
You have to find the "spout", which is a small square plug near the distributor, and unplug it.
Look on your air box, or under your hood for your emissions label info for your timing set.
usually it's 10* BTC, but may be different on your truck.
Set #1 plug to this mark on the damper with a timing light, by loosening, and turning the distributor.
when the lines line up on the damper, lock the distributor down
Plug the spout back in, and you should be good to go.
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Last edited by dustybumpers; 01-11-2015 at 09:22 AM.
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Old 02-09-2015, 01:41 PM
greystreak92 greystreak92 is offline
 
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Replace the ECT (Engine Coolant Temperature) sensor (three-wire sensor typically mounted near the water neck). The symptoms you describe are textbook for an ECT sensor failing with a "stuck cold" condition. The ECT sensor "tells" the ECM (computer) what temperature the engine is at. The computer adjusts fuel-air mixture accordingly. However, if the sensor fails it essentially "sticks" at ONE temperature. If it "sticks cold" it makes hot restarts extremely difficult because the computer "thinks" that the engine is already warm. Hot engines need much less fuel and much more air. Trying to hot restart the truck by dumping that much fuel is essentially flooding it. The opposite "stuck hot" condition makes cold starts damned near impossible. The reason this problem doesn't show up in KOEO and KOER diagnostics tests is because as long as the reading from the ECT sensor is within the range the computer "expects" to see there is no fault recorded. That temperature range runs from about 50 to 195F with 50 being the cold limit and vice-versa.

As for the slow cranking, hot restarts can be that way especially with an aging starter. That could be the result of a dead or dying winding in the starter armature.
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