94 2.3L 2WD NO high idle on cold startup/what to do about two vacuum hoses

[danielwd]

Thanks to all for the many fixes that have been accomplished with your help over the last few months:

- New TPS sensor (solved code)
- Blocked EGR (solved code)
- New ECT (engine coolant temperature sensor) sensor (solved code)
- Replaced all four fuel injectors, cleaned TB and upper and lower intakes with new gaskets throughout (solved rough idle - inability to drive - one injector motor had completely failed)
- I removed A/C system because it hasn't worked in over a decade, and I needed room under the hood to work - specifically to install a brass flare nut on the EGR port off the headers. The evaporator was very much in my way, so I yanked it.

The truck is running great. I have even driven it across town multiple times.

However, I am observing a never-seen phenomenon with my newly repaired Ranger. I have owned this truck since 1994 and never observed it before.

On cold (or warm, or hot) startup, there is about a 1 second high idle and it completely bottoms out to probably around what I am estimating 500 to 600 RPMs.

The IAC valve is probably only a couple years old, if that. I have tested its resistance and it was 9.1 ohms. I backprobed the harness with truck running, and it was about 11.79 V.

I believe, if I remember right, that this may have begun after I changed out the ECT sensor. (I am wondering if it is defective, and I still have the old one - thinking about reinstalling old one to test idle upon startup)

There are no new codes either with key on or key off, it is passing all, except with key on/engine running and that is only because I have the DPFE sensor unplugged because the EGR system is blocked off.

Which brings me to another issue - with the EGR valve now blocked, I have two vacuum lines that used to be hooked to the EGR vacuum solenoid that are still pulling air from the lower intake canal (through the vacuum tree). Of course, the other line from the vacuum tree goes to the fuel pressure regulator. Should I block off these two lines (to and from EGR vacuum solenoid) with heavy duty tape or just let them leak and pull air in? I have tried both blocking off with tape and just allowing them to hang and pull air (I can hear the hiss with even the hood closed), and there has been no detected difference in performance. I can't believe that either scenario of blocking with tape or just letting them hang and hiss would not affect the performance of the truck, but to the best of my knowledge, at the present time, neither approach affects the truck either on startup or on the road.

I got excited thinking I may have figured out the lack of high idle on startup when I tightened the drive belt, and for a couple of seconds, there was indeed a normal high idle, and then...boom...dropped right back down as it has been doing. The excitement quickly diffused. Just how tight should the drive belt be - I am afraid to tighten it anymore - in fact, after tightening, it seems like it idled rough for a little while then mellowed out a bit?

So to recap - what could cause the lack of high idle on cold (or warm, or hot) startup? And what should I do about the two EGR vacuum solenoid hoses - should I tape them off or just allow them to hang and pull air?

Also...should I try and tighten drive belt even more than I already have - could a loose drive belt cause a lack of high idle upon startup?

Thanks so much in advance!!!!!

 

[danielwd]

Also, let me add....there is no maintaining of normal high idling when increasing RPM's manually at the throttle body. There is usually a sort of lag when upping RPM's at throttle body that seems to be lacking. It is like the engine revs up with a manual increase of RPM's at throttle body, but it directly bottoms out.

I have owned the truck since 1994. I know how it works. This behavior is irregular.

Lastly, I did reset the base idle with the TPS at 1 V. The base timing screw has been messed with so many times in the past, that I figured...what the heck, let's reset it. I unplugged IAC, backprobed TPS backing off idle to about 500 - 600 RPM's with TPS at 1V. I also backprobed TPS and revved up RPM's manually at throttle body in an attempt to measure voltage differences. I observed an increase in voltage across the TPS when revving up RPM's at throttle body that made sense with the research I carried out online. I did this with the engine off and with the engine running, and all readings seemed normal.

Thought those details were relevant.

 

[tomw]

I would plug the vacuum lines. They normally did not vent to atmosphere, so you are adding unexpected air to the intake, leaning it out a bit. That could affect idle.
I read all the above, and am not really sure what is the problem, except for the title.

When cold, the IAC should bump the idle for a few minutes. If changing the ECT caused that to not happen any more, I'd swap the ECT once more. The ECT is not on the side of the block above the oil filter, that is the temp gauge or light sending unit. There is also an ACT - air charge temp - sensor mounted on the intake mannifold, which varies resistance by temperature. You can check values/temp using a VOM, and the table from the Ford manual.
tom

 

[Mark_88]

In the warmup cycle one of the things that the older systems had going on was the purge solenoid would open up and allow fuel vapors to be drawn into the intake. This all works great and doesn't cause any problems...unless one of the hoses is off or cracked...then you have an open vacuum leak when the solenoid is open.

I'm not sure how long that stays open but I'd imagine it is a very short time and should not affect the entire warmup cycle unless it is shut down by something like a signal indicating the warm up is done. CTS or something else...then until it warms up you'd have rough idle.

The easy way to test that is to simply plug the vacuum line from the intake to the purge solenoid and you can simply trace it from the driver side inner fender to the engine and plug it up.

If the rough idle disappears completely then you need to check all the hoses from the solenoid to the canister by the rad and then maybe back to the fuel tank.

If the idle does not smooth out then you can at least rule that out of the problem.

 

[danielwd]

In the warmup cycle one of the things that the older systems had going on was the purge solenoid would open up and allow fuel vapors to be drawn into the intake. This all works great and doesn't cause any problems...unless one of the hoses is off or cracked...then you have an open vacuum leak when the solenoid is open.

I'm not sure how long that stays open but I'd imagine it is a very short time and should not affect the entire warmup cycle unless it is shut down by something like a signal indicating the warm up is done. CTS or something else...then until it warms up you'd have rough idle.

The easy way to test that is to simply plug the vacuum line from the intake to the purge solenoid and you can simply trace it from the driver side inner fender to the engine and plug it up.

If the rough idle disappears completely then you need to check all the hoses from the solenoid to the canister by the rad and then maybe back to the fuel tank.

If the idle does not smooth out then you can at least rule that out of the problem.

Mark...thanks for your reply.

I checked the all the air flow components thoroughly. Then all look good. I still question the hose that used to go to the EGR vacuum solenoid. With the EGR system blocked off, I simply removed the EGR vacuum solenoid, along with the DPFE sensor. There is no function for any of it with the EGR blocked off. However, the vacuum hose that draws vacuum from the intake canal of the lower intake (through the vacuum tree) still sucks in air. I simply have it taped off. I am not sure that is the correct thing to do, but there are no performance differences with the hose blocked off with tape or simply hanging sucking air (which is audibly detectable with the engine running - it is a pretty loud hiss).

*EDIT...I just now saw tom's advice on blocking the vacuum lines, which I have done - the truck seems to idle a bit better with them blocked*

The truck is running great - it idles smoothly and drives great. The continuous glaring issue is that there is no high idle upon startup with a cold engine. It is in the 30's here at night, yet in the morning upon startup, there is no high idle at all.

I am confident I have a stuck open thermostat. The truck barely warms up, not even making it to the "Normal" mark. The truck historically has always ran somewhere between the "N" mark".

I have no heat, but the top radiator and heater hoses are hot to the touch after the truck has been running for a while. I haven't tested the bottom radiator in terms of how hot it feels. I guess I need to do that.

The next interesting question is whether a stuck open thermostat could cause the loss of high idle on cold startup?

I am still suspicious that the idle air control valve, which isn't that old, is the culprit of the loss of high idle on cold startup even though it passed all backprobing and resistance testing with the multimeter. I may swap it out with a new one and return it if it's not the issue.

Then I'm going to tackle replacing the thermostat and top and lower radiator hoses - the top hose looks like it has a weakness at the top because it looks like it is just barely bulging.

 

[danielwd]

I would plug the vacuum lines. They normally did not vent to atmosphere, so you are adding unexpected air to the intake, leaning it out a bit. That could affect idle.
I read all the above, and am not really sure what is the problem, except for the title.

When cold, the IAC should bump the idle for a few minutes. If changing the ECT caused that to not happen any more, I'd swap the ECT once more. The ECT is not on the side of the block above the oil filter, that is the temp gauge or light sending unit. There is also an ACT - air charge temp - sensor mounted on the intake mannifold, which varies resistance by temperature. You can check values/temp using a VOM, and the table from the Ford manual.
tom

The problem is that I think the truck may be stuck in open loop or the IAC valve is not operating correctly, although it has passed all the tests I threw at it with a multimeter.

My truck has always high idled for many minutes on cold startup. But after the extensive work I carried out, it no longer does it.

Upon startup with a cold engine, it starts, idles high for about one second, then the bottom drops out and it falls to about 600 RPM's (I have no way to gauge the exact RPM's - I'm guessing).

I also now feel confident I have a stuck open thermostat because the temperature gauge is not rising nearly as high as it used to. I also get no heat in the cabin even though the top radiator hose and heater hoses are both hot to the touch.

I think my first course of action is to swap out the IAC with a new one from AZ. If that doesn't fix the lack of high idle on cold startup, then I'll return it.

Then I'll tackle replacing the thermostat and radiator hoses, of which the top hose is looking rough.

Thanks for all the input. I'll post back with results in a while.

BTW, I agree with your assessment about blocking the EGR vacuum solenoid hose and have taken your advice. The truck idles better with it taped off.

 

[danielwd]

Also...tom...my air charge temperature sensor looks to be bolted into the lower half of my air box. There is no code for it, but it may be the likely culprit of my loss of high idle on cold engine startup.

I guess it may be possible that if that sensor is screwed up, the computer may not be able to detect the temperature of the air upon startup, so maybe if that sensor is broke, I could lose my high idle upon cold engine startup.

I may test it tomorrow and report back. Thanks for that bit of advice.

 

[danielwd]

Swapped out IAC valve under warranty at AZ. It was 2 years old. After swap, the truck started with normal high idle with a cold engine. It's weird because the IAC valve passed backprobing and resistance tests, but it still was not operating correctly. Strangely, after installing the new IAC valve, my fan blower suddenly corrected itself and now works at all speeds. It was only working on high speed only, leading me to believe that the transistor block was broke. Strange that swapping out IAC valve fixed this issue.

I am still not getting heat in the cabin, so I am guessing that my thermostat is stuck open.

 

[Mark_88]

Good to hear the IAC fixed the high idle...it is possible the valve was stuck and because that is physical you won't know unless you take it out and inspect. I cleaned mine up when I had idle problems but it didn't help...

The blower motor works off the resistor in the heater box. They tend to get really rusted and eventually fail but are not hard to replace. I found that heater boxes that had the resistor mounted parallel with the rad (older with no AC) were more prone to corrosion than those mounted on an angle...no idea why.

Edit: forgot to mention that it may start to work again (the fan speeds) but best to grab a spare before it goes out again...I've had one stop working and then start again when I hit a bump...then went out a few days later.

They are the same used in many Ford vehicles so a trip to the junkyard can score one for next to nothing...even though they are less than $20 last time I priced one.

Are the hoses into the heater rad getting hot or are they tepid? If they are hot you may have a blockage in the rad core itself...but the return hose would be cold if that were the case...or maybe both hoses if there is no flow though them. Backflushing might help in that case...not hard to do and if it hasn't been done for a while then maybe a good time to flush the whole system...if you need to change the T-Stat that would be a good time to flush the entire system.