can you tell me what this does?

Well, for the past few weeks I have been chasing down a vibration in the 1100 to 1500 rpm range. I figured for a last ditch effort I would give it a tune up, and see what happened. On my 88 xj, I changed the o2 sensor, plugs, and IAC this morning. I started it up and let it idle for a bit. Now, I always had an issue of the idle jumping to about 3000 rpm for a bit, then I would gas it, and most of the time ot would come back down. So I thought maybe changing the IAC would fix this. Well, the vibration I think either went away, or I just don't notice it from under the hood, but now I can not get it to idle under 1100 RPM. I noticed that when I plugged this vaccuum line, the idle would drop. But, then the idle jump up to 3000 rpm, and is now staying there. Can you tell me if this may have something to do with it.



This is where the line is coming from



this is where it is at



And this is on the fender. I noticed that only one vaccuum line is hooked up to this. Should there be two, and should the vaccuum line come from the missing one?

Only one of your pics is showing up, but the symptoms of the high idle, and the location of the vacuum lines, my guess would be the MAP sensor - is the check engine light on?

Whooops - now I'm seeing more pics - definitely looks like the map sensor. I can't tell about the center pic, but if you have an open port, I'd put a cap on it to eliminate the vacuum leak, especially if that is a t-port with the MAP sensor. If it doesn't read the manifold vacuum right, it can cause havoc on the idle, and the air/fuel mixture as well ....

okay, got the pics fixed. The MAP sensor is hooked up, I haven't changed it yet though, I didn't want to spend another 70 bucks if that would not cause it.

I thought the MAP sensor was on the firewall. Oh, and no check engine light...

Might be a vacuum controller soleniod - I'm getting blind/stupid in my old age ...

figured it out.....egr solenoid......

That is the EGR solenoid - there's a "supply" line, an "actuation" line, and a "vent" line (that really isn't a "line" at all - but that's what the missing hose is. Or isn't. Take that as you will.)

Not all of the ports in the "tree" are used, either. If you aren't sure, let the engine idle and spray every vacuum connection with carburettor cleaner in a fog - if the fog gets sucked in and the idle jumps, you've found the leak. Here are a few tips...

1) The MAP sensor is a standard GM part - this little factoid can save you a few bucks. Fact is, most of the engine control sensors are standard GM - the only one I can think of offhand that you can't substitute is the HEGO sensor, since RENIX used a variable resistance unit rather than a voltage generating unit.

2) I've found a lot of vacuum leaks caused by loose manifold bolts - on inline sixes in general. This is common, and is partly due to the long sealing surface, and partly due to the bolts being expected to hold aluminum and steel at elevated temperatures - which tends to "wiggle" the bolts loose. I'm working on a kit that would offer a better fix for that little annoyance. As a side note - I retorque my manifolds as part of an annual inspection - I think I've always found at least two loose bolts!

3) The Nylon vacuum lines can crack without showing at first. That's where the "fog" trick gets handy.

4) Watch the MAP sensor connection at the throttle body - that one works loose sometimes as well. It's a two-bung connector, one of them is blind and retains the connection. A little RTV Black on the "blind" bung usually fixes that little annoyance.

5) I've also traced vacuum leaks to a faulty EGR gasket - so that's something else to keep in mind. I always coat these with RTV Copper when I change them.

I'm not trying to say my esteemed colleagues are incorrect in their diagnosis, I just want to make sure that you know everything else you should check (in case they are...) Most of these checks can be done quickly, and for less than $10 (or whatever a can or two of carburettor cleaner can cost you.)

5-90

Thanks much 5-90. Since I was doing a tune up I decided to go ahead and redo all of my vacuum lines. That's why I need to know what the heck that was. You are right, I found cracks after pulling them off that I never would have seen. The motor purrs like a champ now. Now If I could only get rid of that friggin vibration

jephs422 said:

And this is on the fender. I noticed that only one vaccuum line is hooked up to this. Should there be two, and should the vaccuum line come from the missing one?

Click to expand...

If I'm following correctly, the missing vacum line isn't. The opening with no line attached(above three orange-ish wires) is a breather. Look into the opening and you will see a filter.

Thaks, after looking a little further, I found a vacuum diagram and realized it was a filter. I hooked up the missing line to the air box. Now it runs right. I am still getting the high idle on occasion, but it has been that way since I got it. It runs good as of now. I think next check I will pick up a MAP sensor and see if that fixes the high idle...