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Weird starting issue

kaidenshi

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1988
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Hi folks, I posted this at broncoiicorral.com but I know there are more eyes on this forum. I hope you don't mind me crossposting it, I'm just at my wits' end with this truck. Original post below:

Hey folks, I've been having a weird issue I hope you can help with. 1988 BII with manual and 4x4, replacement 2.9L engine with 30k miles pulled from a wrecked 1986. The issue only shows up in cold weather (less than 60 degrees), in warm weather it runs fine. What's happening is it will start but immediately bog down and run rough, like it's not timed right or it's misfiring constantly. It's a steady "thrum, thrum, thrum" like a cammed big block Chevy. Eventually the truck will warm up to operating temperature and it will suddenly run fine. If I try to drive it while it's bogging like that it barely moves and backfires like mad. Once warmed up it drives like it should. In warm weather, I can start it and it never bogs down, and I can drive it immediately with no issues.

I've replaced a ton of parts over the last two years I've been restoring it, including most sensors and the IAC. Whole new exhaust system including O2 sensor and cat, new ignition system -- plugs/wires/coil/TFI/battery/starter/alternator, everything except the distributor itself and it has a new cap and rotor. In fact, I had tried replacing the distributor but the new one never worked no matter how many times I installed it, yet the old one worked fine. New cooling system including radiator, hoses, cap, thermostat, coolant temp sensor, and water pump. New fuel tank and sending unit/pump, new injectors, new fuel pressure regulator.

I'm thinking it has to be an electrical issue, two things are pointing me to that. One, the alternator is the last part I've replaced and when I replaced it, it improved but did not completely get rid of the issue. Before the new alternator, it would bog down immediately after starting and not run correctly until warmed up. With the new alternator, it starts and runs fine for a minute or two before bogging down, then running fine again after warming up. Two, when it's running rough and I turn on any power draw that runs through the chassis electrical system (headlights, turn signal, windshield wipers, heater blower) it bogs even worse and sometimes dies. Once warmed up, power draws don't affect it. Any power draw that runs directly off the battery (LED light bar) doesn't affect it at all.

I haven't yet tried the engine management computer, and I've found and fixed a couple of brittle/broken wires in the engine wiring harness with no change in performance. This gremlin has me pulling my hair out and wanting to get rid of the Bronco, but I would rather fix it and drive it than let it go. If anyone has any ideas I'd be grateful.
 


kaidenshi

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So I replaced my ACT sensor and it somewhat helped but didn't fix the issue. I decided to get out the Innova code reader even though it isn't turning on the Check Engine light. KOEO was fine, CM was fine, KOER returned a code 41 - O2 sensor lean condition. I do have an exhaust leak at the passenger manifold, I just don't understand how that would affect cold idle (I thought EEC-IV ignored O2 readings until the engine is warmed up?). I'm going to tackle the manifold if it ever stops raining here.
 

holyford86

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check all your grounds, including the oxygen sensor ground bolted to the back of the passenger side cylinder head
 

kaidenshi

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@holyford86 The O2 sensor ground was not connected, I don't know how I missed that one when replacing the engine. I didn't have a grounding point on the back of the passenger side head and it wasn't long enough to reach the grounding point on the driver's side head or firewall, so I bolted it to the rearmost upper intake bolt on the passenger side. It's showing continuity with all my other grounds so I'm assuming that's a good enough grounding spot. It didn't help my issue any though.

Speaking of the other grounds, they all seem fine but I think I'm going to replace my negative battery cable and engine-to-firewall ground strap anyway. I was originally planning to do that this coming weekend but my wife decided she wants glass packs installed on her Crown Victoria, and I'm a smart man who knows what's best for me. ;-)
 

RonD

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I would check the 5volt Reference Voltage coming from the computer.
It powers air temp, coolant temp, MAP, and throttle position sensors

If power draw effects engine operation it could be this 5v is dropping lower which, of course would, effect sensor readings to computer

Test voltage with engine off key on and then turn on head lights or Blower to see if 5volts changes, should be stable


You can pull computer out and take off the top cover, there are 3 blue capacitors that can leak after 20+ years and cause odd problems
Picture of what you should see here: http://www.auto-diagnostics.info/ford_eec_iv

The capacitors are cheap and can be replaced
It could be the voltage draw is causing internal computer issue, the 5v reference voltage is just a starting guess


Also with engine running check voltage at battery, every thing off
Then turn on lights, blower, ect.......and see what voltage does, should drop instantly then come back up to what it was, thats the voltage regulator doing it's job

Now do the same test but use the Red wire on IAC Valve as positive, it gets 12v from EEC relay when key is on, so thru the truck wiring not battery direct
 
Last edited:

kaidenshi

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1988
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Engine Size
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Transmission
Manual
Thanks @RonD, I'll add your post to my ever-growing list of things to narrow down. I deal with stuff like bad capacitors on old boards as a matter of course at work, so I'm familiar with what to look for. I greatly appreciate the info and the link!

I think even though my grounds "look good" and show continuity, I'm going to just redo all of them; remove each connection, sand or wire brush them on the lug and the contact point, and snug them all back up. I think I'll take a second look at my fusible links too, I already had to fix one that had virtually disintegrated from corrosion.
 

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