Actually, after going 13 rounds with this mid-'80s Toyota MR2, we found there were more than just two "hard knocks."
My friend had owned and enjoyed the clean little two-seater for more than a decade, and even had the engine replaced a couple of years ago because of an overheating problem that had spiraled out of control on a long trip. But recently, the car had developed a rolling idle that nobody was able to fix, and it appeared that the overheating problem that destroyed the first engine was beginning to resurface.The owner took it to a Nissan dealer she knew and trusted, but they weren't really interested in tackling the car's problems because they immediately farmed the job out to an independent shop. While driving the Toyota back to the dealer, it belched some coolant out the recovery tank overflow. A second look by the independent shop indicated that perhaps the car might have blown a head gasket. At this point, the owner asked me to have a look at the little machine.
First look
In the service bay, we checked the coolant level, and it was only slightly low. On this cooling system, the filler cap is in the engine compartment on the opposite end of the car from the radiator, which is up front between the headlights. Several yards of plumbing are necessary to make this arrangement work, so there is a draincock-style air bleeder on the top left side of the radiator and another bleeder on the thermostat housing in the engine compartment.After filling and bleeding the cooling system, we let the engine run with the filler cap off. I watched
for the coolant to boil and gurgle those sickly, sweet-smelling combustion gases that are a sure-fire indicator of head gasket problems, but it never happened.Replacing the radiator cap, we monitored the temp gauge and kept a close check on the hoses for any indication of premature pressure. We didn't see any blatant evidence of a blown gasket.
The "hunting" starts
As the engine reached operating temperature, the rolling idle started. It was cyclic and rhythmic, sweeping up and down about a thousand rpm every couple of seconds. It was vaguely similar to the normal sound of a cold L-Jetronic feedback fuel system looking for the proper idle speed, but on this engine it was sharper and more pronounced. Cold hunting on older Lambda-equipped Bosch systems is apparently normal, as noted in VW technical service bulletin 93-06, which states that when cold, "the idle speed should hunt for approximately one minute ... this is an indication of a properly functioning air-fuel system trying to compensate for varying air-fuel mixtures." Whether this is normal on the MR2 remains to be seen (at least by me), but it obviously shouldn't fluctuate between 800 and 2,000 rpm when the engine is warm. Loosening the radiator air bleed again, more bubbles came from the radiator, and the idle hunting suddenly evaporated. The little car was idling at a smooth 800 rpm. This reminded me of older Isuzu engines that do crazy things at idle because of air bubbles in the block. I thought that maybe I had moved an air pocket away from the Engine Coolant Sensor when I bled the radiator. We drove the car around town, and let it idle for a long time that day and the next day. However, it didn't belch out any coolant or even attempt to run hot, and, as far as we could tell, the rolling idle was gone. The radiator fan appeared to be working fine, so I called the owner to pick up the car.For whom the bell tolls
That evening, the bell tolled. The MR2 owner called and said she had driven about 14 miles, stopped for gas and coolant belched out again as she was turning in at the gas station."Did it run hot?" I asked.
"No," she said, "not according to the gauge. It just left a long stream of antifreeze when I turned in, and it was doing that rolling idle thing again. We left it at the gas station, and we're going back out there after the car is cool to drive it back over to you at the college. And please don't tell me we need a radiator cap. We've already had two different shops replace the radiator cap."
When we saw it again, the MR2 was definitely low on coolant, but as far as we could tell there were no leaks. After we refilled and re-bled the system, it again refused to overheat. However, the hunting idle did return, and we decided to delve a little deeper into that problem. We started with the Toyota four-wire TPS, which consists of a potentiometer and an idle switch.The top two terminals on the TPS as it is mounted on the engine are IDL1, which is a signal wire, and E2, which is ground, or return. See Figure 1. With the throttle closed, the idle switch is closed and IDL1 should read zero volts. The red lead in the photo is back-probing IDL1, while the black lead is probing E2. The voltage should remain at zero until the throttle lever moves no more than 0.232 inch from the throttle stop screw. As the throttle is moved away from the idle stop, the voltage should jump to 12 volts at IDL1. The third pin down on the TPS is VTA1, and the bottom terminal is reference voltage.
I loosened the TPS and rotated it slightly to open the switch, and the idle stabilized at about 1,700 rpm. The stable idle lead me to believe - albeit tentatively - that the TPS idle switch had oxidized contacts. But before buying a new sensor, we opted to drive the car around for awhile. For two days there was no more hunting, but a 14-mile test-drive to caused the pesky little machine to belch coolant once more.
Priorities
It seemed more likely to belch coolant driving around town than on the highway, but the cooling fan was operating correctly. I decided to contact a friend with a lot of Toyota experience who might know where to find the weak links in this chain. I described the coolant-belching problem, and he said he had seen three MR2s do exactly the same thing. On each one, a worn out water pump impeller was the culprit.He reminded me that the pump has to move coolant through yards of plumbing to and from the radiator. I decided it was worth a look. It took two of my students two hours to get the water pump off, but we found that it had been replaced with the new engine. Another two hours and a new gasket were invested in reinstalling the water pump. We then removed the radiator and spent $55 to have it rodded out.
The coolant belching was completely cured after we reinstalled the radiator, but the annoying idle roll was still there. The information available on ALLDATA gives a procedure for checking the electronic idle speed control, but this MR2 is equipped with a mechanical wax element Auxiliary Air Valve (AAV). Thinking maybe an air bubble was trapped in there, I loosened the coolant hoses leading to the valve and made sure there was good flow. The idle hunting was more consistent now, and nothing I could do would change it. I could start the MR2 warm and let it idle for about five minutes, and the idle hunt would kick in just like clockwork. At this point, I noticed that someone had tinkered with the Airflow Sensor, so I decided to pop the plastic cover off just to have a look ...
Sticker shock
I noticed a scorched place on the board. The sensor should read 20 to 400 ohms at rest and as high as 3,000 at the end of travel. Our measurements (taken between E2 and Vs) started at about 150 ohms, jumped to 400, then lazily drifted back and forth between 400 and 600 all the way through its travel. What I was seeing didn't look right, and that disgusting rich-smelling exhaust added support to the faulty Airflow Sensor hypothesis.Tapping into the O2 signal line, I found the sensor wouldn't read higher than about 450 millivolts (mV). Since it appeared to be original, I ordered an aftermarket replacement, and with the new sensor the voltage bounced between 550 mV and 1.5 volts. At least the new O2 sensor was telling the truth, but for some reason the PCM was still dumping too much fuel.
Convinced that the tainted Airflow Sensor was causing this rich condition, I checked on the price of a new one and was stunned to get a quote of nearly $800! Calling salvage yards and checking the Internet produced no results. I didn't want to install an expensive part like the Airflow Sensor and find that we still had the same old rolling idle.Jumpering the fuel pump terminals in the Airflow Sensor connector (the pump won't run unless the vane moves and lets the switch close), I decided to fire the car up with the air inlet tube removed. Even with the Airflow Sensor out of the loop, the idle was still unstable. This eliminated the Airflow Sensor as the cause of the rolling idle, but I felt certain it was causing the rich idle condition. Still, the car ran well and because Alabama doesn't have emissions testing, my friend could probably get by for a while. I did notice that if I goosed the throttle with the Airflow Sensor removed, the engine would bog - but that was to be expected. With the air inlet tube removed and the engine running, there was one more test I wanted to perform.
Bingo!
On domestic powerplants, I always like to know if the air that's making the engine run too fast is going through the throttle body or hissing in through a split hose, an intake gasket or some other point downstream of the throttle plate. If you cover the throttle body with your hand and it either tries to suck your hand in or kills the engine, all the intake air is going through the throttle body. If the throttle plate really is closed, idle problems are usually in the Idle Air Control valve. If the vehicle continues running fast with your hand smothering the throttle body, you can look somewhere else for an air leak. In this case, it tried to suck my hand in, and the engine almost died. I was getting closer.I noticed that the hole where the AAV takes in air sounded like a hurricane, so I partially covered it with my finger. As soon as I did this, the idle hunting stopped and the engine leveled out at less than 1,000 rpm. The wax element AAV was sticking in the fast idle position, and the PCM was doing whatever it could in a vain attempt to bring the idle speed under control. Intermittently, the valve would suddenly decide to work properly and the idle would stabilize until the next cold-start.
A new air valve from Toyota was installed, but it didn't solve the problem entirely. On the next cold-start, the MR2 idled at 1,500 rpm instead of 2,000, but as the vehicle warmed up, it began to hunt again. The problem wasn't bad and most drivers would have probably accepted it, but I wasn't about to let it go like this.
The hoses feeding the coolant to the new AAV were hot to the touch, but that didn't mean coolant was flowing to the valve. When I disconnected the hoses from the valve and jumpered them together with a piece of clear tubing, I found that there was almost no coolant flow. Coolant should flow quickly through the AAV all the time, even before the thermostat opens. The pressure in both hoses was obviously equal, so I installed a 10mm cadmium-plated nut as an orifice downstream from one of the hoses. Restarting the engine, we saw coolant go screaming though the clear plastic tube. With the hoses connected to the AAV, the engine idled down to spec within 60 seconds and stayed smooth.None of my service information says anything about the Toyota wax pellet AAV, and I haven't found any techs who know anything more than I do about this system. I have no information stating that there's supposed to be an orifice in this system, but maybe a special hose was left out when the engine was changed. It's not the first time I've seen an orifice used to control flow, and I'm satisfied with the fix.
