94 Ranger Revving Issue

[Tractor Dan]

Here are the specs
94 Ranger
3.0L
Rear wheel drive
Manual Trans

The original engine went bad in it. Blown crankshaft. My mechanic dropped a yard engine in. Not sure what year the donor vehicle was but the IAC had a date sticker with 1990 on it so leads me to believe it was a 91 or 92 motor. The old engine i believe had EGR and the 91 or 92 motor didn't. I base this off the fact that my mechanic swapped the upper intake off the blown engine over and plugged the hole where the EGR valve would bolt up.

When i put the clutch in it revs up. Not sure what the rpm is as i don't have a tach. When it does this it's only if the clutch pedal is depressed and as soon as i shift to the next gear and let the clutch out it goes back to a normal idle. It also does this if i leave the clutch pressed in and try to hit the brakes to slow down or take it out of gear and try to slow down using the brakes. So i can't figure out what's going on with it. I did get a check engine light tonight and if i was reading the flashes of light correctly on the dash board using the method to retrieve codes i was getting codes 31,32,33. I'm not sure if i was reading them correctly though.

So what i'm wondering is could this be a computer issue where the computer is having issues since it's not seeing any EGR on board what so ever? I know OBD1 is not as sophisticated as OBD2 but i would assume that if an ECU was programmed to see a component and it didn't especially something like EGR it would have issues? So i'm trying to decide if i should find an ECU for a 91 or 92 motor once i determine which it is that is set up for my spec and no egr and try that? The computer systems are still new to me. I just finished one class at school on them that was more or less an introduction to it. The next class gets deep into the theory and logic behind them and whatnot.

 

[Spott]

If I understand correctly, EGR has a pretty subtle effect on the engine. My 2.9l was swapped in from an 86 or 87, and has the EGR plumbing and correct computer, but the truck is an 88 and lacks the EGR wiring in the harness. I haven't gotten around to fixing that problem yet. According to my research, enabling the EGR will slightly improve my cruising efficiency and slightly improve emmisions. It will not have a significant effect on the idle, and the engine will run just fine without it. The problem does not involve your engine computer, with any reasonable amount of probability.

Your problem is a simple high idle. Whenever the engine is allowed to run free, it tries to idle very quickly. When you re-engage the drivetrain, the wheels slow the engine down to match the speed you're driving.

The engine computer automatically adjusts the amount of fuel to match the amount of air that's going through the engine, so apparently you're getting too much air in. It's much like having the throttle pedal pressed down a bit, all the time. Likely causes of this are (in order of likelihood) a vacuum leak, an Idle Air Control valve stuck open, or a throttle valve stuck open.

Take a can of starting fluid and spray short bursts in various places around the engine while it's idling. If the engine revs up a bit when you spray in a certain spot, you know the starting fluid is getting drawn into the engine by your leak near where you are spraying. That should help you locate the leak.

If you can't find a leak that way, unplug the IAC. If the engine lugs way down or stalls, then your IAC is good. If it stays running strong, then your IAC is stuck open, and needs cleaned or replaced.

Let us know what you find.

 

[Tractor Dan]

Thanks for the input. I'll do some looking at it this weekend. What you said about EGR does make sense. I had replaced the IAC back in February so i'm guessing that's clean but i'll check that and the other things you suggested.

 

[RonD]

Yes, you read the codes correctly, 31, 32, 33 are all EGR valve codes, no EGR sensor or Valve would produce those codes.


EGR system is not used at idle RPMs, it is used only under engine load to lower cylinder temperature which prevents pinging and lowers NOx emissions.
Contrary to popular opinion EGR system does not change power output, it would be best to re-install it.
3.0l was prone to pinging on low octane fuel, pinging can melt spark plugs and eat away at head gaskets, worse case it will melt exhaust valves and pistons.
Computer setup for EGR system will probably run engine with lower MPG and power output because EGR system is not working.

For high idle issue:
First check if IAC valve is working.
Warm up engine then while it is idling unplug connector from IAC valve, idle should drop down to 500rpms or it might stall, if this happens then IAC valve is working and for some reason computer is setting a high idle so not a vacuum leak.

If idle doesn't change with IAC Valve unplugged then computer has closed IAC Valve all the way and engine is still idling too high so most likely cause is a vacuum leak.


Engine computer(PCM) uses it's own sensor to "see" what the engine temp is, this is the 2 wire ECT(engine coolant temp) sensor.
There is a similar looking 1 wire SENDER that is used for the dash board temp gauge.
When you start the engine the computer looks at the ECT sensor first, if ECT shows cold engine the computer runs "choke mode", this sets a high idle, about 1,100rpms, and computer opens injectors longer, so runs engine rich.
As ECT sensor warms up the computer starts to lower the idle and run engine a little leaner.
It should take an engine 5 to 8 minutes to go from cold to fully warmed up, just below 1/2 on temp gauge.
If yours is taking longer then thermostat is not working correctly, and idle can stay high because computer thinks engine is still cold.
Proper t-stat for 3.0l is 192degF or 195degF, don't use a 180 or 160 t-stat.

 

[Tractor Dan]

I'm not sure why my mechanic didn't swap the EGR over. Probably because he didn't want to swap exhaust manifolds and whatnot. I'm not too sure though. I might have to go talk to him but after our last conversation when i got tired of waiting for him to do a job after six weeks and took the truck else where, i don't think he wants to see me for a while.

I know the thermostat is good, we just replaced that shortly after swapping the engine in. Wasn't getting above 160F. It had a 192F thermostat but that wasn't functioning correctly. So dropped a 192 or 195 in whatever the parts store had and it has been great ever since. I'm wondering if the ECT could be bad. As i think about it my 97 that i had, the ECT went bad. it looked fine from the outside but as soon as i went to loosen it the plastic turned to powder.

Guess i'll check the stuff suggested. Pull plugs and look at those. Check the ECT. Figure i have nothing but time so eliminate things one by one. I also have access to Mitchell Pro Demand through school so that should help to find specs. Thanks for the input folks.

 

[Tractor Dan]

Unhooked the IAC twice. Once when i got to school engine rpm lowered but without a tach i can't say exact rpm's. However based off sound i would guess between 500 to 1000 rpm's. The second time i done it was at a roadside rest waiting for a tow truck, one of my tires decided to have the tread separate and low and behold my spare is the one that came with the truck in 94. I feel like the truck is doing that to me. Got a used tire though. Back on topic it done about the same idle drop as at school.

I'm going to check everything else this weekend and this coming week. I'm going to look at cleaning that throttle body the best i can, not quite sure the best method since it's built into the intake.

 

[Spott]

Unless the rpms drop so far that the engine is on the verge of stalling, then your IAC test indicates that you have a leak somewhere (also sounds like your IAC is good).

Look at your throttle valve; the butterfly should close all the way. Then do the spray test.