I have a 1988 F150 that I am in the process of rehabing. I have fixed many small porblems but still have a few to go. One problem is that I still can't get proper idle and I don't have vacuum to the EGR valve. In going through the elecrical prints I have discovered that there should be a diode accross the leads of the idle air bypass valve. I have checked and it is not there or elsewhere in the wiring. I have checked dealers and parts stores and knowone is familar with this diode. The plug for the IABV has been replaced in the past and I'm betting that when the harness end was cut the diode was in the bundle and the person replacing the plug did not realize it was there. Does anyone know what the value or rating of the diode is supposed to be? I'm betting I could find a replacement at radio shack if I knew what to look for.
I could hope that with the diode in place that might help the vacuum problem as well. Hay if you're going to dream, dream big.
Owen

 

Can't help you with the diode problem, but I can tell you that it is not responsible for no vacuum to the EGR valve. [Which I think you already knew. ]

It shouldn't be difficult to trace the problem. Simply start by tracing the vacuum line from the EGR valve back to the source. Inspect for breaks/cracks- trace the line all the way thru, back to the vacuum tree on the manifold. I'm not sure on the specifics of the 88 system, but it should look something like this:

EGR valve > EGR vac solenoid > vac canister [there will likely be some tees between the solenoid and canister that will lead to other components- make sure to inspect those lines as well] > vac tree on manifold.

If you don't find any visual breaks or cracks, get a can of carb cleaner or starter fluid. Using a straw to get good aim and direction/isolation, shoot a little bit at a time at each fitting you see and along the lines themselves. Be very careful and do not spray a lot at once- a little starter fluid on a hot exhaust manifold could make for a very bad day. If you notice the engine rev a bit after you spray a certain spot, that could be the only leak or one of many.

Here is a page to more info on the EGR vac solenoid/regulator. It will provide you with more specific info on how to test/troubleshoot it. Ford Fuel Injection » EGR Vacuum Regulator (EVR) When I had EGR problems on my 91 F150 the regulator was the problem.

Good luck and keep us updated.

 

That is a suppression diode that is built into the device. The IAC has one as well as as the EVR. These coils have enough of a back EMF when the voltage is from removed them that it can damage the drivers inside the PCM. Placing a diode across the coil keeps the voltage/current spike from feeding back to the PCM. Very common practice in analog electronics.

Your issue is most likely a faulty EVR. Whether or not there is a diode across the IAC is irrelevant to your situation.

 

I replaced the EVR but still have a problem the idle speed is to high. The voltage accross the Idle Air Valve is 12.24 instead of the 10.5 specified in my manual. I have checked the funtion of the TPS and it appears to be working. I did have a code 23 when I started this project but no codes are present at this time. The engine idles at about 25mph in gear. Sometimes it surges. What controls the voltage to the idle air valve?
Owen

 

The IAC is modulated off/on by the PCM, it is typically not a straight DC voltage. The most common cause of a high idle is a vacuum leak followed by a faulty TPS. A bad IAC usually exhibits a rolling idle/surging as well as stalling from time to time.

Could you gives us more details of this statement "I have checked the function of the TPS and it appears to be working."

For reference here is how the IAC works: Ford Fuel Injection » Idle Air Bypass (IAB)

 

The voltage check was made with the key on and engine not running. When I checked the TPS the voltage went from .46 - 5.2 volts from idle to full speed. Reference voltage with key on was 4.6.
I tried a new IAB with no improvement.

 

Unless your reference voltage is above 5.00 VDC there is no way the TPS signal is going to go that high. Normal VREF is in the 5.00 VDC range, +/- 0.2VDC at most.

What are you using as a ground reference? The next question is what did you use to measure the TPS? The key item to look at when measuring the TPS is for signs of a varying output caused by dirt/corrosion on the internal wiper of the sensor. It is a little tricky to look for that with a digital meter, but as long as you know what to look for it's not a show stopper.

With the power to the IAC removed is the idle RPM above 500-600 RPM? If it is there are two thing to look for. The first is verify if the idle stop screw has been moved, if that appears undisturbed you most likely have a vacuum leak.

 

I have found the vacuum leak. It is on the drivers side rear of the upper intake manifold. I used the spray method mentioned in the reply from DBGrif91. Nothing showed up the first time I tried it the other day but it showed up big time this afternoon. I bought the new gasket set and will start work on replacing the gasket tomorrow morning. This may not be the whole problem but it sure appears to be a big part of it. Thanks guys, I'll keep you posted.
Owen

 

I hope the gasket fixes your problem. Thanks for the feedback. Let us know how it goes!

 

I replaced the gasket and stoped that leak. Once I got the engine running I tried the spray test everywhere and no more leaks. However the engine is still idling to fast and will go from idle to full speed on its own. If I reach in and unplug the IAC valve the engine will die. I have tried a new valve with no improvement. Any ideas? No codes showing.
Owen

 

I do not see where you answered my previous question about how you measured that TPS and VREF signal. Your results do not add up.

A surging idle is usually a combination of a vacuum leak and a sticky IAC. You have replaced the IAC, so one item is eliminated. The vacuum leak is still suspect, although that theory is running a thing in light of you stating the engine dies when you disconnect the IAC power feed. I still advocate using a hand-held vacuum pump/gauge to test each vacuum circuit. I find the spray method can be deceiving at times.

Your TPS test results are the more troublesome to me.

 

There is no way you can have TPS voltage higher than the Reference voltage.
And if the Reference voltage was 4.6 you have problems with the 5 volt regulator inside the PCM Computer.
Sounds like you have the Reference voltage and TPS voltage swaped.
Even so 5.2 is to high for the Reference voltage.
Also .46 is too low for the TPS closed throttle.

 

I used a digital voltmeter to measure the voltage. I am willing to try something else if you have a better idea. I have replaced to old ratty vacuum can with a new one and replaced EVR with one from a working engine no difference. I have also tried the PCM from a working engine no difference.
What are the two small control valves to the right of the EVR used for? They are not show or refferenced in my book.
I do have a hand pump vacuum tool and have used it to check the interity of the vacuum lines. That is how I found the broken green vacuum line going to the EGR. At idle if I apply vacuum to the EGR with my tool the idle drops off big time which I am told is normal.

 

They are the A.I.R. Bypass & A.I.R. Dump solenoids.

 

They obviously dump and bypass but what do they control. I don't have an explanation of A.I.R. in my book.