ScuzzMonkey

I've been having a problem with my '94 4-cylinder 4x4 for a few months with the idle surging when I am on the brakes at stoplights before it is entirely warmed up (but not when it is completely cold). I spent a lot of time combing the forums for an answer and thought I had finally got it when I found a post suggesting I warm it up, cover the AAV outlet hole in the throttle body with some duct tape, and see what happened. I did that, and when I covered the hole, the idle dropped back down and it didn't surge. So, I pulled the AAV and cleaned it, as suggested, but that didn't help. I figured, hey, maybe it's just bad, but it's a $200 part so I wanted to get a pro to take a look at it before I replaced it.

So the mechanic calls back today and says it's a blown head gasket. Now, I just changed the oil a week ago, and it was not even remotely milky. I know that isn't necessarily the only symptom, but I haven't noticed any other problems besides the idle surge. So my question to all you learned folks is, could a blown head gasket produce the idle surge? And if so, would my experiment covering up the AAV hole still have appeared to fix the issue, even if it was the head gasket that was the problem?

Thanks!

my-4x4

My dad had the same issue with his 88 3.0, it happened like 4 yrs ago...and it turned out to be a screw that could be adjusted....I know I wasn't much help but i'll look to see if I can find the email

ScuzzMonkey

I should mention I guess that I did check and try altering the setting of the idle adjust screw, but it didn't fix the problem.

Stuwy123

iTrader: (1)

I've had two things cause idle surge.
- Air in the lines feeding the Auxiliary Air Valve. Which could be low coolant, or a puncture/air leak in the hose feeding the valve.
- Blocked coolant line feeding the Auxiliary Air Valve. There's a nipple right beside the thermostat housing. Pull the hose off, and unscrew the nipple. I found a wad of gunk in there. Clean it out and see what happens.
There's lots of info on fixes for the idle surge.

ScuzzMonkey

Hi Stuwy,

Yeah, I flushed that little hose out and cleaned the nipple out, too. I have tried most of the things that people have said can cause the idle surge, I guess what I really want to know is, can a blown head gasket cause it, too? Or is this mechanic off base? I followed the suggestions for diagnosing it, and they seemed to point to the AAV, but could a leaky head gasket produce the same result?

Junkers88

When it's between the "warm" and "cold" you will sometimes get a surge. I do in mine once in a while. The reason for it is that the engine is almost at operating temp but not quite, when you press the brakes the ECU will cut the fuel since it knows that you are stopping. Well the idle will come back up a tad since the engine isn't quite warm yet.... the ECU cuts the fuel... idle back up a little.... ECU.... etc.

The same thing happens when folks have the idle set too high. If it does not do it when the engine is warm then you don't have an issue.


Now if it is idling high due to the IACV (AAV) being bad then yeah you probably want to replace it. Mine is shot and the rig idles at 2200 when it's cold, I just let it sit and warm up before I drive off.

edit. I would bet serious money that it is NOT the head gasket.

4Crawler

iTrader: (3)

x2 on too high an idle, turn it down and see if the surging stops. Why, see below:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

ScuzzMonkey

I have tried setting the idle every which way but loose; at the moment, the screw is in almost as far as it will go, so I am sure it's not too high. I mostly suspect the AAV right now, I'm just trying to figure out why the mechanic is saying it's the head gasket, because if that's true there's no point in replacing the AAV.

Stuwy123

iTrader: (1)

Yes, a blown head gasket can cause idle surge.
The exhaust/cylinder pressure can force out the coolant, pushing air into your coolant, causing the idle surge. This will really only be noticeable if the coolant overflow bottle is pouring out coolant basically non-stop.

Stuwy123

iTrader: (1)

Put tape over the auxiliary air valve.
Does it still do the idle surge?
Once the engine is warmed up fully, (upper rad hose gets hot/warmer), set the idle.

Do a coolant pressure test. If you lose pressure, you're losing coolant somewhere, if you can't find it (in the dash, or on the engine), it's going into the cylinders. But if this is true, the overflow bottle will be constantly overflowing.

Is your coolant full?
Did you check both hoses coming off the AAV?
How's your thermostat? Anything stuck inside the housing?

4Crawler

iTrader: (3)

You can test the HG leak either by a chemical test on the coolant looking for exhaust gas contamination (NAPA sells a kit for around $25). Or pressure test the cooling system and look for rising pressure with rising engine revs.

greencactus3

mine ended up being a vacuum leak at the plenum gaswket

snobdds

Have you tried to pressure test the cooling system? Have you tried to do a compression test. Those two will tell you if you have a blown head gasket.

As mentioned before if you have surging when the brakes are applied, that means the throttle is too high. It might be you have a faulty TPS.

ScuzzMonkey

Thank you! That's exactly what I wanted to know.

I did do the duct tape check, as you mention later, and it does NOT surge when the outlet is covered. So if it's the head gasket causing it, would that still be true? That's why I am suspicious, because I wouldn't think it would.

I haven't been losing coolant, it's still full and hasn't been topped up in forever. I did check both the hoses coming off the AAV and cleaned them when I cleaned the valve; did NOT check the thermostat, but it has not been overheating.

I don't necessarily disbelieve the mechanic, although if I could get hold of him again I'd like to find out what tests he ran to make his diagnosis, but I just hadn't seen any other symptoms of a HG problem, so it seemed like an odd thing to tie to the idle problem.

Stuwy123

iTrader: (1)

Check your coolant. That's the first thing you should do.

Did you have good coolant flow from the two hoses connected to the AAV? Meaning, did they leak coolant when you took them off?
If they didn't, you're probably low on coolant or have a blockage further down the line from the AAV.

The problem could still be your HG even if you put the duct tape over the AAV air passages. The AAV is what is causing the surging. The valve is opening and closing. You need to find out why. The AAV is connected to coolant system, that's what you need to check next. Obviously the AAV is working properly because it's opening and closing just fine (all it's supposed to do).

ScuzzMonkey

Okay, thanks Stuwy, that helps a lot. I am getting good flow in the hoses to the AAV, they've both had coolant in them every time I've taken them off, and they get good and hot when the engine is warm. So it sounds like the HG is probably my only problem at this point (unfortunately, an expensive one!)

Cheers, and thanks everyone for the advice.

Stuwy123

iTrader: (1)

The hoses will have coolant in them but if it doesn't flow, it will surge.
I had the idle surge problem when only the one line was plugged.
I cleaned out the line and free-ed the plug, then it worked.

Was coolant FLOWING out of both lines?


Did your mechanic do a coolant pressure test?
I wouldn't deem it to be a HG unless you are 100% sure, not just guessing. That would be an expensive guess.

Head Gasket kit from Engnbldr is $64, throw in the steel timing chain set (if you haven't already) w/ an oil pump for $86.
It took me 3.5 hours to remove the head, another 8 or so to clean everything up and put it all back together.
Have the head decked and the blocked checked. DO NOT SKIP THIS PART!!! Or you will be doing another head gasket in a short amount of time.