I have been trying to chase this down for awhile, but not having any success.
My truck has a lot of issues and I'm slowly fixing them as I get money and time. I am not using it as a driver right now and it is sitting in our shop. (Mostly annoying me at this point,ha)
The problem is when I start it, it revs too high. Only does this on start up and runs good otherwise.
It revs up to @2300 and stays there for a couple seconds, then drops to around 1300 or so for a bit when cold, then down to around 750. If its warm or hot it revs up to @2300 for a couple seconds and then down to 750.
For the numbers I am relying on the factory tach at this point, which I understand may not be especially accurate, I just got the low amp probe for my genisys scope module so I should be able to tap into the pip signal to check accuracy.
But I believe its fairly close from the idle speed, driving rpm, and general sound.
The engine really needs mains and an oil pump among other things, but I want to fix this high rev on start issue before I pull it.
When you combine the high rev and need for bearings and a pump it sounds like its going to explode every time it starts.
I do have a napa gold filter on it, but the PO used whatever filter was cheap.
I have a disassembled 4.9 from an '89 in the bed, and the 5spd trans that came with it, seems to be a pretty much exact match for mine so I have some handy replacement parts.
My TPS had a small flat spot off of idle, so I replaced it with the one from the other engine, fixed the dropout, but no change on the high rev start.
I cleaned both of my idle air assemblies (the one on the truck, and the spare), no change on high rev, both appear to work correctly. When I unplug either one, the engine speed drops.
I adjusted the throttle stop, by setting it where it will just barely idle when warm, with idle motor unplugged - around 500 rpm which gives a tps voltage of .78 (or right about there, don't remember exactly as it has been a couple weeks) and an idle speed of around 750 with motor plugged in.
I also tried setting it to .96 volts, gives a slightly higher warm idle, but not a big difference.
Neither method made a difference in the high rev issue.
I have the non adjustable tps by the way.
I can unplug the idle air motor and start the truck without it reving high, but if its cold I have to give it some gas for awhile, warm it start and run, but just barely.
I reset the timing to 10 degrees btdc with spout unplugged, was pretty close to that already. Drops way back when I plug the spout in, would have to guess at exactly how much as it goes off the guage. Advances when I rev it.
If I don't find an answer in the electrical system, I will check the self test timing change against a light and then check for wear in the timing gears and distributor gear.
I've been back and forth a bit with codes due to my unplugging and changing things, but the only real code I get is for the egr position sensor. I just temporarily blocked off the egr with a thin piece of metal. I didn't have time to change it out that day. Could the computer be suppling extra fuel on start because it believes the egr is partially open?
From what I've read here and on the old ford fuel injection site I gather it wouldn't, but I've been wrong before.
I've checked the temp sending unit both hot and cold and it seems to have the correct resistance.
I pulled the air charge temp sender and cleaned it.
The alternator is getting ready to die, but is still putting out 13.8 or so most of the time. I plan on doing the 3g conversion next.
The battery is brand new as are the lugs and heavy wires. I've cleaned and/or tested the grounds. There resistance between ground and the signal return is good, actually no resistance according to the meter I was using.
I haven't checked all the lines with a vacuum pump yet, but the engine does not act as if it has a vacuum leak, it really runs quite well other than the high rev on start up.
I haven't checked the fuel pressure yet, maybe the regulator isn't returning fuel and its getting way too much pressure?
I bought this truck about 4 years ago and fixed the shifter pins/bushings and welded in some pipe where the PO had cut out the cat. Put some tires on it and used it to haul a bunch of scrap for about a year or so. During that time I also gave it a tune up, oil changes, had to replace the coil, and the computer. The computer is a rebuilt one from advance. The reason I replaced it was because it wouldn't run the fuel pumps. Truck would run fine with the test lead grounded.
Truck had the high rev issue with the PO as well, I can verify this as he parked next to my bedroom and used to go to work at 4 am, with the cut out cat it was, um... noticable.
What else? Oh, the clutch safety switch has been jumped. I have some firewall issues to fix before that will function correctly.
The ignition has been straight wired, basically they cut the two large yellow wires running to the ignition slider, wrapped the ends together and then through a toggle switch mounted in the dash, then just slid the igniton switch to the run position. The starter itself has a push button with wires running all the way out to the solenoid.
I plan on putting this back to the way it should be, but I have to fix the column interlock first. They must have grounded something out at one point as the fuseable link off the solenoid for the yellow hot wires had been replaced with a regular piece of wire, I put a fuseable link back in.
Not the best setup, but I will fix it. I did notice that if I kill the engine manually, say by blocking off the throttle body, and I restart it by just hitting the starter, without touching the igniton toggle it doesn't over rev, just revs up to about 1300 or so, then settles down.
So if I don't break power to the ignition it seems to restart as it should.
From the diagrams I have it looks to me as if there is a constant power feed from the ignition to the computer, do you think interrupting this feed by using their toggle switch I'm causing information in the computer to be lost which makes it rev up high like that on start?
I've been trying to get it to start by using the ignition slider (just with a screwdriver at this point) and have been running into issues. I had no power at the solenoid on the red/blue wire when trying to crank. I tried sending power directly from the clutch interlock and still got no power. After chasing wiring around for awhile I discovered the red/blue wires running to where there would be neutral safety switch if I had an automatic had what looks like half a jumper across them, I fixed that. Then I had power to the solenoid and could spin the starter by applying power to the wire at the clutch switch. Still not from the ignition slider. The slider itself wasn't in great shape, I had to tighten the potmetal tabs to get full connection, so I replaced it, still didn't work.
About this time it redeveloped a problem I had forgotten about, the starter wasn't engaging. Decided to fix that before it tore up the flywheel. So now it has a new starter.
I'm sorry for going on at such length, but I figure more information than you need is better than less. If anybody has an idea of what might be causing this I would very much appreciate hearing it. Maybe somebody else has an ignition wired like this and has the same issue?
If nothing else, this may help somebody else someday, I have managed to figure out a lot of things that do not cause the high rev on start issue, lol.
Ummm.... my '89 straight 6 revs to 1500RPM when cold, and stays there for a good 3-5 min, or even more depending on how cold it is, and in summer, it doesn't have high idle, just starts and revs at 700-800RPM cold in summer.
2300RPM is way to high on a cold engine! Glad mine doesn't do that!
I was thinking all those high cold revs were a contributing factor to the bearings being loose. That and the PO not using oil filters with good drainback valves, and of course the mileage. Although I don't know what the mileage actually is, says 72k on the odometer. I'm pretty sure that isn't its first trip around though.
I forgot to add in my orginal post that sometimes the truck only revs to 1500 or so on start. Not very often and its pretty random. Might be cold, might be hot. Might not happen for a week or more, might happen twice in one day. Might go to 1800 or so, and then back to 2300 on successive starts, might just go right back to 2300 the next start. Might happen after I've changed something, or might happen out of the blue. Hard to say how often it does this, maybe once out of every 50 starts, 100? Really too random to put a number on.
I want to make everything right before I pull the engine, so I guess I will continue to correct issues and perhaps one of them will make a difference.
If not when I get to the point where everything is fixed except the bearing issue I will have at least eliminated a lot of possibilites.
Once again, I thank you for taking the time to reply to my post.