Red Dodge Diary

A second story about the 1988 GMC Safari van comes to mind. Now, I was not the primary driver of this vehicle, as I had my own truck to drive. However, one day when I was driving the van I noticed that the engine idle was just a little higher than normal. Or so I thought because initially I wasn’t sure, I remember thinking it seemed higher than what I was expecting. The next time after that when I drove it, there was no doubt. The engine idle was increasing slowly over weeks. It got to the point that you had to be ready to drop it in gear so it wouldn’t take off on you. Apparently I should have begun my troubleshooting well before that point. As it happens, I had ordered a complete shop manual with the vehicle.

Many new cars come with a page in the owners manual that allow you to order (at nominal cost), a shop manual which contains infinitely more information than any owners manual.

Throttle Body

 

This book became quite useful at different times. I was not very experienced with computer controlled engines at this time, so I had a learning curve to negotiate. OK, we have an “idle air control” or IAC device that could be at fault in causing the high idle speed. Hmmmm. Makes sense. I bought a new one, installed it on the throttle body which is the thing that used to be called a carburetor.   Well, it looks like a carburetor, but operates differently so it’s called a throttle body. Technicalities. So did the IAC replacement cure the high idle?  No.    Why should it? It was a best guess item to replace, since the computer threw no codes at all, no “check engine” light, all I had was my shop manual.

Oxygen Sensor

Another item to check was the oxygen sensor mounted in the exhaust pipe under the van.

Hmmm, looks ok. Better replace that too. No difference. I’m sure you’ve noticed at this point, I may not know exactly what I’m looking for, and you’d be right.

I made a few educated guesses based on the description of how the system works, such as “What controls idle speed on this engine?” The computer only knows what it’s told by it’s various sensors which are out there collecting data. Water temperature, oxygen content in the exhaust gasses, things like this. It takes the data and decides if the air/ fuel ratio needs adjustment.

Idle Air Control

Something was telling this computer to make an adjustment. One that caused a high idle condition. This particular year model of vehicle did not have the multitude of sensors that perhaps today’s engines do, therefore I was running out of things to check. It was now that my experience on non-computer controlled engines helped. I thought, well, if this was a normal engine without all the crap on it, what would be causing a high idle? Everything else being the same, that is, no work was performed on this engine that may have induced a problem, like modifications, the first thing that I thought of was, “air leak in the induction”.

Extra air leaking into the intake system will lean out a fuel/air mixture, meaning too much air to the amount of fuel entering. This results in a few things, one is a higher than normal idle to a point. If it gets too lean, it just quits running. Another product of a lean mixture is higher exhaust temperatures. So now I’m looking for air leaks. Hmmm, hoses everywhere, have to inspect those to see if there’s a cracked one, or disconnected, allowing air to enter.        Nope, those look good, but not to worry.

I have a little trick I use now and then looking for air leaks.  Take a spray bottle and put some water in it. Go around spraying water in a stream on various hose connections and around flanges where things mount, like the throttle body. If you suddenly hear a sucking sound, and notice a change in the way the engine is running, you’ve found something to investigate. Now don’t go hosing down electrical stuff, or spraying water into openings where it doesn’t belong. Any water sucked through a leak from your little spray bottle isn’t going to hurt a thing. This is exactly what I did and behold!! I found a leak at the base of the throttle body where it mounts to the engine. I removed the throttle body from the engine, and knew immediately I had found the problem.

The gasket between the TB and the engine had deteriorated so much that a fairly large portion of it was missing.Without the gasket sealing off the surfaces between the TB and engine, air was being sucked right in, the computer knew something was wrong because the exhaust gas sensor told it so. The computer in an attempt to take care of things, schedules in a little more fuel to make the exhaust gasses correct, and juggles the idle air control, but it has gotten past the point where it can compensate for this air leak.

End result, a new five dollar gasket totally cured the problem. What a sweet sound, starting it back up, hearing it idle down. All is well once again.

Throttle Body Gasket