Hello,
Have a question concerning testing EGR valve. According to the test you are suppose to disconnect the IAC and remove & plug the vacuum line at the EGR valve. The start the truck and let it idle. What I am not sure of is what is suppose to determine if the EGR valve is good or bad. It idles but lower than usual and doesn't die. Any help will be greatly appreciated.

 

This is how the test works.

You disconnect the IAC so the computer can't control the idle speed.

You disconnect the vacuum hose to the EGR so no vacuum is applied to it.

You're supposed to hook up a tach, start the engine and note the engine rpm

With a hand vacuum pump you slowly apply 5-10 inches of vacuum to the EGR valve. The idlle speed should slowly decrease as the valve opens.

On my 88 4.9 when 10 inches of vacuum is reached, the engine stalls.

According to my 88 manual the engine speed should decrease 100 rpms or more or stall.

Remove the vacuum from the egr valve and the engine rpm should return to the original idle speed plus or minus 25 rpms.

If the idle speed doesn't change, it would indicate the EGR valve is not opening or could be clogged.

 

When you say lower than usual, are we talking less than 680 RPM? That is base idle for the 5.0. You'l get away with +/- 50 RPM, however, faster will make an auto tranny clunk pretty hard when you shift it into R or D. Mine runs about 620 without IAC, and about 710 with. I don't worry about without, because you won't drive it very well without, especially with a manual. It would stall every time you shifted without the IAC. 710 suits me just fine, and 30 over isn't enough to worry about with a manual tranny.

The test described is a good test, if you suspect one isn't opening. Not opening will do little to affect performance, however, besides throw a code and make the engine run a little warmer on the highway. Not closing, however, will cause all kinds of grief such as rough idle, idle surge, stalling, light throttle miss, and on the list goes. I've even blocked mine off for a couple 200 mile trips using a piece of steel between the EGR flange and the plenum when I was hunting intake leaks. With it blocked in this fashion, the valve still moves, and the sensor detects position change, so no code is thrown. There wasn't enough change in engine temperature to notice on the stock gauge. It may have been a needle width higher. The valve will wear with age, and will inherently start causing some vacuum leak around the spool. This is usually a slight flutter or sputter when idling, or under light throttle at low RPM, but is seldom noticed under load or WOT conditions. My guess is because of the reduced vacuum in the intake, and the fact the EGR closes when a significant throttle change is made.

The whole purpose of the EGR is to cool the burn in the cylinders by displacing some of the air with non-combustible exhaust, thus reducing NOS emissions, and generally making the engine run a little cooler. The emissions benefit appears to be the primary reason, because like I said, it really didn't make much difference in temperature. I did notice a slightly more positive throttle response from cruising speed to WOT when passing. Not enough to warrant a permanent block, though. My guess is the response came from not having to wait for the EGR to close. Older SD systems are notorious for the lag in throttle response anyway.

 

Still haven't had a chance to apply vacuum to it & don't have a tach to hook up but have a feeling it will be pertty obvious if I have a problem. Have a feeling there is a problem there for I am experiancing all the symptons (Lack of power, irrattic idle, stalling at idle, peridoc missing under light throttle). It appears the only other thing is blocked catalytic converter which is also possible but want to test the EGR Valve first.
The other thing does anyone know if it is common for the EGR pipe to get blocked down at the intake? Really don't want to have to take the plenum off but if I have to I will.

 

I doubt a clogged egr passage would cause your problem. A leaking egr valve would.
Here's a article that may help you diagnose your problem.
http://www.aa1car.com/library/converter.htm

 

Umm, I don't think the EGR's gonna block up at the intake. The port on that thing is HUGE. Even if it did plug up, it wouldn't cause the lean miss symptoms you are describing. Now, if you're talking about the steel tube that goes down to the head, umm dunno really. Never heard of it plugging up, but that doesn't mean it can't happen. If it plugs up on that side, it's possible that it could exacerbate a vacuum leak, but still wouldn't make any difference at idle. The EGR is closed at idle and WOT, and has some delay when the throttle steadies at cruising speed based on coolant temperature. So, plugged on either side probably won't bother your performance (except maybe improve it slightly). It may cause a slightly higher operating temperature at highway speed, but I blocked mine intentionally (same symptoms as you), and the effect was fairly negligible. The worst problem you'll have if it's blocked will be Nox if you have emmissions inspection. I don't have to worry about such inspections, but you'll also find you have somewhat better highway fuel economy if the EGR is working right.

What WILL happen, though, if you're getting lean miss on some cylinders and the others are running rich because the ECM is trying to correct for it, is you may get a very significant soot/carbon buildup on the back of the throttle plates, and at the base of the throttle body. This will certainly cause idling issues and poor performance. Remove the hoses to the air box, open the throttle wide open, and look in there with a flash light and mirror. You'll see the build up easily if it's there.