I am still having the cold idle problem on my 1988 2.6L Trooper. If you want a very detailed description of the problem behavior and what I have already done to try and fix it, you can look at some of my previous posts.

Cold Idle Problem

Specifically, my question today is if there is one (or more) temperature sending unit(s) that tell the ECM that the engine is cold or warmed up? Since it runs perfect once warmed up, it seems that perhaps something is telling the ECM that the engine is warm when it is actually cold.

The Idle Air Control Valve appears to be working fine. The IAC has strong vacuum at start-up which decreases to minimal vacuum once warm. I can not find any vacuum leaks. I have used propane and starting fluid everywhere I can see a vacuum/air intake hose and still have found no leaks. Just for some background, this all started after the Mass Airflow Sensor (rebuilt Hitachi unit) was replaced along with the Fuel Pressure Regulator. This was done to fix a no start/flooding problem. I am not looking to beat a dead horse, but I can't let this go. It is driving me crazy and I am afraid that this problem could end up causing some other more major/expensive problem. Thanks in advance.

I added a link to my previous post. Hopefully it works!

How about a link instead of making us search???




There are two water temp sensors in the lower intake manifold. The 2-wire sensor is for the ECM. The 1-wire sensor is for running the temp gauge.





You lost me here. An IAC is a computer controlled idle valve. The 2.6 doesn't have an IAC valve. And what do you mean by it has "strong vacuum"?

Cold idle is kicked up two ways on the 4Z. First is the Aux Air Valve. This is bolted underneath the intake manifold with a hose running to it and to the intake tube in front of the throttle body. When cold, it lets air pass and a bi-metallic strip heats (electrically) and closes it off.

The 2nd is the Thermal Valve. This is the tree looking valve on top of the t-stat housing. It passes air to the intake manifold when the water is cold, and re-directs it to the fast idle solenoid as the water heats up.

The fast idle solenoid then will open when you have the A/C on to kick the idle up again for that. I'm not sure if it's there though if you don't have A/C.


So, an idle problem when cold likely won't be an ECM/sensor issue. (I suppose it still could be a fuel/MAF issue though). It's controlled mechanically by the AAV and the TV. Both of these have hoses that run to the intake tube in front of the throttle body, so by doing a cold start and alternately pinching them and seeing their effect on idle, you should be able to figure out which one is not doing it's job.


--Dan

there is 1 temp sending unit that goes to the computer. take a look at this for the explicit location.

Done......I think. I have not linked before and was unsure how. I think I got it right.



I saw in the manual that it showed several temperature sensors on the Intake Manifold, so this takes care of one of my future questions.



Sorry. I have seen it called the IAC as well as the Auxiliary Air Valve. I did not know there was a difference. <img src="/forums/images/graemlins/confused.gif" alt="" /> Yes, I mean the Auxiliary Air Valve. The big hose that attaches on the side of the throttle body has strong vacuum at cold start and then the vacuum decreases as the engine/valve heat up.



I am at wits end so the sensor just seemed like a remote possibility. The Mass Airflow Sensor (and the Fuel Pressure Regulator) was replaced with a rebuilt unit just prior to the problem developing. If the MAF sensor was faulty, why would it run perfect once warmed up?



I thought the Thermal Valve only opened to allow additional cool air in when the system starts to get hot, such as when the AC is on? I did not know it has anything to do with cold start. Could this create the symptoms described in my previous post?

Since the problem started immediately after having the MAF sensor and the Fuel Pressure Regulator replaced, I am planning to go to the U-Pull-It yard and see if I can find a MAF sensor in good shape to switch out. I also plan to get one of the Thermal Valves and one of the Temperature Sending Units. I am going to switch out one at a time and see if, by some miracle, I stumble onto the problem.

Ok, I gotcha. Vacuum there would indicate the valve is open. Does it stumble worse when you pinch that off?





I'm only thinking that there could be an issue with fuel as well, though I think it's not very likely. The ECM runs it richer when cold than when warm. It's possible that the MAF could be an issue, but it's not very likely since it runs fine when warmed up.





Doh! Brain fart. You've got it right. The opens to the intake manifold when the water starts to run too hot. That kicks up the fan speed for more cooling. The A/C kick-up is still done through the fast-idle solenoid that the valve feeds.

I'll see if I can add any confusi...er, ideas after I get a chance to browse your previous post.

--Dan

I had a similar problem that seemed to resolve when I cleaned out the EGR. If the thing hangs open at closed throttle it'll drown the idle with excess exhaust air just the same as if you pushed the diaphragm on the EGR. I found that not only was the EGR port on my 2.6 loaded with carbon, so was the back pressure tube that goes into the bottom port of the transducer. I had to take that tube off and clear the hole with a small drill bit. Keep in mind that the operation of the EGR is cut off on a cold start by the thermal valve in the underside of the intake manifold. If that valve has failed in the open position that will allow the EGR to work when the engine is too cold to accomodate the exhaust gas and choke your engine.

Also check your air intake tube directly over your valve cover gasket. Mine had a hole the size of a 50 cent piece in it on the bottom right where you couldn't see it.

I might have posted this in one of your earlier posts but I'm too lazy to go look.

Joe

In addition to all of that stuff, it'd be a good idea to replace all the old vacuum lines.

I am not familiar with the "Thermal Valve" on the underside of the manifold. Perhaps it is called something else? Where is it positioned and what does it look like?

I have gone over every inch of the air intake/vacuum hose system with starting fluid and also an unlighted propane torch (more control than starting fluid spray). I have not found anywhere that appears to respond and indicate a leak.

At the U-Pull-It yard I noticed some sort of valve with vacuum hoses attatched on the tire side of the manifold. It was right next to the single wire temperature sending unit for the temp guage. What does this little unit do? It had two vacuum hoses going to it ( I assume one in and one out). Since it screws into the coolant flow in the manifold, I assume it is a temperature controlled vaccum valve. Is this correct? Could this little unit have something to do with the problem?

Argh, your the one who keeps taking Vancouver U-pull it Isuzu stuff. I go to those routinely, and last time took a whole bunch of 2.6 throtte body, and intake manifold parts. When you figure out what's bad, let me know. I may have a U-pull it spare. <img src="/forums/images/graemlins/smile.gif" alt="" />