Air temp sensor drama continues

rickb78 · Post by **rickb78** » 17 Jul 2011, 08:57

The original beginning of this tragedy started when my wife's 91 740 stated bucking, hesitating, very little power on the way to church. After church she couldn't get it started and got a ride home from church. I was out of town and got home the next day. I drove down to the church. Her car started right up so I drove it home, around 5 miles. At mile 4 it started bucking , hesitating, and gave almost no response when I pushed down the gas pedal. But I limped home. The OBD indicated code 1-2-2, air temp sensor. I left the hood of the car up overnight and the next morning the battery was dead. Would not take a jump. The battery was almost 6 years old. Went to Wal-Mart, got new battery.

1. With a new battery the voltage to the air temp sensor measures a consistent 1.86, which is too low. I did not want to tear the head off the connector to check the voltage directly from the wiring. I did peel back the rubber sheathing 2-3inches from the connector that contains the wiring (grn/brwn-blk) and found nothing but the wiring which looked new, i.e. nothing suspicious. I see on the wiring diagram for this Regina system that the green wire goes to the injector ECU while the brn/blk wire goes to the inlet manifold sensor. Is it going to be necessary to physically trace both these wires back to their respective connections to the ECU and the manifold sensor to see if the problem originates there?

2. There is a cream colored, small diameter hose that attaches to the underside of the manifold pressure sensor. I followed that hose to its other end, which has a u-shaped rubber connector at the end that slides over a small metal tube. There was a small hole at the u-joint I thought might be the problem. I pulled the rubber connector off, turned it around and put it back on, making sure the hole part sat over the hose so there would be no leakage. Started the engine and it just idles, doesn't hesitate or sound like it's going to die.

3. While it was idling I pulled the rubber hose off the pressure regulator to see what effect it might have. Nothing.

I feel like I'm in a Kafka nightmare here.

Post by **jimmy57** » 17 Jul 2011, 09:08

Take off air cleaner as if you are servicing the filter. Inspect the door in the bottom where the hose from exhaust msanifold and the black plastic duct from from next to radiator connect to the box. There is a thermostatic device that should have the door closed to hot air tube and fully open to the cold air duct from next to radiator. If not then then remove the thermostat and use a screw run in over edge of the door to hold it closed to the hot side. Put it all back together and see if it is repaired.
The t-stats fail and stick to hot air side. The hot air in texas 105 degree heat makes the IAT sensor go WAY hot and the EFI gets wonky with extra hot air temp.

rickb78 · Post by **rickb78** » 17 Jul 2011, 17:20

Thanks Jimmy I'll do that later this evening to see. It's 103 right now at 1823. I really appreciate the response.

rickb78 · Post by **rickb78** » 18 Jul 2011, 04:51

Ok, I removed the air box from the car. The first thing I noticed was how clean the filter looked! I haven't changed it in like forever. The second thing I saw was the door (flapper thing) to the cold air duct was closed and the hot air door was open. The tube the thermostat sits in is really tightly fitted into the air box. I sprayed some stuff I used to help unstick frozen fittings on the fuel filter I changed recently onto it then called it a night- still 98 degrees outside at 2100.

Have another question. After taking out the thermostat and wedging the hot air duct closed, will the cold air duct remain open simply from the inrush of air from the radiator or do I need to wedge it open?

Post by **jimmy57** » 18 Jul 2011, 08:25

I always run a drywall screw or other slender sheet metal or wood screw through edge of door into wall of filter housing and secure it open to cool air.
The hot air off manifold cover reduces emissions when car is operated in temps near or below 0F and it also assures no throttle freezing in freezing temp operation. In Austin I think your safe.

rickb78 · Post by **rickb78** » 19 Jul 2011, 03:28

Thank you Jimmy. Am still trying to remove the tube the thermostat is in from the box w/o cracking or breaking it. Will let you what happens.

rickb78 · Post by **rickb78** » 20 Jul 2011, 19:29

Finally got the air box thermostat out of the air box then closed the vent to the that aluminum tubing coming off the engine and wedged open the flapper to the cold air duct adjacent to the radiator. Then I put the air box back into the car, put the filter back in, the clamps, re-attached the air temp sensor, and then started the car. It was a rough start but started and I let it idle for 10-15 minutes. I noticed the check engine light went off...hmmm. Also, the engine seemed quieter. I shut it off then started again. This time I goosed the engine. It hesitated alot sounding like it was going to die. This sounds like the same initial problem. I pulled the hose off the intake air pressure sensor back by the OBD and the car died. Put it back on then started it. Great, the check engine light is back on. I'm sitting in the driver's seat in the carport with the window down, it's 101 degrees, I feel like an idiot, but I swear the engine is definitely quieter. What the heck, I pulled out of the carport and tore off down our driveway, which is about half a mile to the front gate. Now it did hesitate like there's something going on with the injectors yet it did not buck with a sudden power loss as it did when the problem first appeared. At least that was my impression. It's hard to tell when you're zooming down the driveway so I turned right at the gate then pushed the pedal to the floor. It gave one heck of a buck or two then it took off- way more power than I ever recall with this car, like it was a turbo and I have owned a 740 turbo, so I know. I went about a mile slowed down and turned around, floored it again which produced a big hesitation-like buck then took off. Once past that hesitation it really took off.

The engine has definitely improved but we're not there yet. I have to respond to the check engine light and that hesitation on acceleration. But I don't know what to do next. Any further suggestions?