Fans and Idle RPMs

  • 2006 GMC YUKON
  • 5.3L
  • V8
  • 4WD
  • 160,000 MILES
I recently bought this SUV and took it to my mechanic and they found a couple different things, the first was a misfire on cylinder three which threw off a engine light. They changed the spark plug wires and that help but they said that it still has a small misfire at idle speed. They recommended I have some sort of engine detergent added and see if that works, but it was going to cost too much, $800.00. Second they said that the car needed antifreeze and the computer recommended a thermostat. Now I have the truck back and the check engine light is still on but it goes off, second I have run the truck for thirty minutes at idle and the temperature is 209 and check with temperature gun and it shows 187 at the top of the manifold where the coolant goes into the engine. The fans seem to run pretty much all the time once operating temperature is reached, is that normal? And it idles at seven to eight hundred rpm's which makes it feel like it wants to take off. Is that the right rpm's for that engine?
Do you
have the same problem?
Tuesday, December 27th, 2016 AT 1:08 PM

1 Reply

First of all, no computer "recommended a thermostat". That may be a message your mechanic entered into his diagnostic equipment, similar to "car wash recommended". If he was reading the diagnostic fault codes, those never ever say to replace parts, or that one is defective. Fault codes only indicate the circuit or system that needs further diagnosis or the unacceptable operating condition. When a part is referenced in a fault code, that part is the cause of the code only about half of the time. First we have to rule out wiring and connector terminal problems before we spend your money on a part.

209 degrees is perfectly fine. 195 degrees is the rating for the most common thermostats, but for many years, GM has run their engines much hotter. On some models the electric radiator fan does not turn on until the coolant reaches over 220 degrees. If your fan is cycling on and off, it is working.

Idle speed is set by the Engine Computer. You will observe that when you start the engine, the speed shoots up to 1500 rpm's, then comes back down within a few seconds. That "idle flare-up" proves the idle air control motor is working and the computer has control of it. After that, the computer will adjust speed based on load, coolant temperature, and ambient air temperature. "Load" basically means when you shift into "drive" or "reverse", which loads the engine down. 800 rpm's is typical for a warmed-up engine. 900 might be normal for a cold engine. The computer will also increase idle speed when it is about to cycle the AC compressor on. Some vehicles also increase engine speed at idle when you turn the steering wheel, to increase the output of the power steering pump and overcome the load it puts on the engine.

I have never heard of cleaning an engine for $800.00, and I doubt some "mechanic-in-a-can" is going to solve a misfire. There are already so many very high-quality detergents and other additives in even the cheapest gasoline formulations, that it is doubtful adding even more is going to help. You need to have the cause of the misfire accurately diagnosed and repaired. If some carbon buildup is causing a misfire, why is not affecting all the other cylinders? Replacing spark plug wires is a good start, but worn spark plugs is a better suspect.

Another point to consider is GM has had a huge problem with mismatched injectors for many years. Chrysler buys their injectors in flow-matched sets from Bosch, and failures are extremely rare. Most other car manufacturers also buy or build their injectors, and match them in-house. GM grabs a handful of injectors out of a large bin and stuffs them into the engine as it comes down the assembly line, with no regard to flow-matching. With high mileage, there will be one or two that have a flow rate slightly reduced from the rest. That results in a slightly lean fuel/air mixture for that one cylinder. The excess unburned oxygen going into the exhaust system is detected by the front oxygen sensor as a lean condition, but that applies to that entire side of the engine. The engine computer does not know that only one cylinder is responsible. It just adds fuel to the fuel metering calculations for all the cylinders on that side. Your mechanic would see that on a scanner as high positive fuel trim numbers for that side of the engine. The corrections might make that one cylinder run better, but the other three would be getting too much fuel. That unburned fuel will burn in the catalytic converter where it can overheat it and melt the catalyst if it gets bad enough.

The solution is to buy a set of rebuilt, flow-matched injectors. There used to be a company near the Indianapolis Speedway that listed the top ten injector-related running problems on their web site. Eight of those applied only to GM cars and trucks. The owner's name was Jim Linder. He used to put on very high-level classes around the country, usually for hundreds of people at a time. I was lucky enough to have him address a little group of fifty people at my community college a few years ago. It was a full day-long school, and almost all he talked about was the problems his company was solving on GM products. There was such a high demand for his injectors, he was buying them from shops, schools, and salvage yards. At one point they were paying $25.00 per coffee can-full. Unfortunately he is retired, but you can still do a search for "Linder Technical Services".

The number one problem on his list was for elusive misfire fault codes on GM vehicles, that defied diagnosis. The clue that you might consider was to switch two injectors on your engine; one being from the cylinder with the misfire. Erase the fault code, then see if a new misfire code sets for the cylinder you moved the suspect injector to. Of course this should be done after all the normal misfire causes have been investigated. Do not reach for the obscure solution before you looked at things like spark plugs, EGR problems, vacuum leaks, and things like that. Any hint is you usually will not feel a misfire caused by mismatched injectors, but the Engine Computer will detect it. Most of the time you can feel a misfire caused by a spark plug.
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Tuesday, December 27th, 2016 AT 1:54 PM

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