Mechanics

HIGH IDLE ISSUE AND INTERMITTANT SHIFTING CAUSE IAC, TPS?

1999 Buick Century • 170,000 miles

Hello, I have a 99 Buick Century and recently I have experienced very high idle speeds and erratic shifting issues. I have cleaned the IAC valve, and did notice my throttle body was rather dirty on the inside. I had cleaned the throttle body with a cleaner, cleaned the IAC, and started the car. The idle was still high. I disconnected the IAC, the TPS, the air box sensor, the sensor on the air intake tube and the EGR sensor, I thought the car should have shuddered or something, so it ran high still. I connected all the sensors back, started the car and still high. My next thought is to disconnect the IAC alone, then the TPS alone. Is there anything I am missing? I have seen others with the same problem, but have not seen any sort of resolution. I thank you for your assistance.
Dennis
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Dlvaughn1963
September 24, 2012.



Here's an interesting design issue. The replacement PVC line is not sufficiently sealed where a secure plug was installed and the hose diameter is different on both sides of the secure plug. The hose has come loose at both sections of the plug. I think the fix for this is either a tubing splice or a single piece hose. It seems the leak is here. When the engine is running, and the hose pieces are held in position the idle returns to normal. So my temp solution is possibly using high temp sealant over the entire hose joint. More to follow.
And the issue of the hissing is when the engine is running and the sound is at or near the evap canister. I've not heard this sound in the four years I've had this car even prior to all this recent engine evap work.

Tiny
Dlvaughn1963
Oct 1, 2012.
Some hose clamps maybe?
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Saturntech9
Oct 2, 2012.
I can try some clamps. It seems that whenever I either move the line the idle goes to normal then after a while the engine surges and goes back high. If I'm at a stop and restart the engine the idle is fine. The hose ends where they go into this secure plug t has some sealant that looks to have gone bad so any serious vibration cause it to lose seal. There seems to be no other leaks at the engine and electrically everything else is ok. When the engine transitions from normal to high idle the engine violently shudders as if to shut off.

Tiny
Dlvaughn1963
Oct 2, 2012.
Gentlemen, thank you for unintentionally saving me some time. My mom has an 01 century as a secondary car. A few months ago I did a full engine rebuild due to some blown gaskets, even polished the heads. It ran like champ and was getting 32mpg highway up until last week. I have the same problems, taking the throttle assembly right now. Hers is throwing a code for the iac but I replaced that when I did the rebuild, with mass airflow sensor and tps. Mileage has been slipping, idle getting rougher, and I too have noticed what sounds like a vacuum leak but, again, I replaced most of the lines. Very frustrating. Keep me posted if you find a solution, I will check back and let you know if I find a fix on this one. Best of luck!

Tiny
Greghoward
Oct 4, 2012.
Use some clamps or get a smaller hose and some lube to get them on.

Saturntech9
Oct 4, 2012.
I think my problem is the EGR valve, it sounds like that's where the hissing is coming from, and it's the only part in that area that I didn't change during the rebuild. Buying it in the morning, will report back tomorrow

Tiny
Greghoward
Oct 4, 2012.
Isnt your egr valve a linear one meaning its electronic not a diagram type?

Saturntech9
Oct 4, 2012.
Bad hose is my situation and clamps won't work. More to come

Tiny
Dlvaughn1963
Oct 4, 2012.
Smaller hoses and lube have you tried that yet?

Saturntech9
Oct 4, 2012.
Because of how the hose is constructed a home made remedy will not work. Where the two lines come together in a "t" it actually vacuums from the valve cover through to the purge solenoid valve, the line is obviously bad. However, this morning I had disconnected the booster vac line and started the engine (of course I forgot I had disconnected it) plugged the pcv hose back into the original purge solenoid valve and the evap line to the lower connector, and plug. With the booster vac line disconnected, it repeated the horrible starting and idle runs I had been experiencing. When I reconnected this line back to the booster, the idle immediately went high and stayed there. When I shut off the engine, restarted it, the idle was slightly higher than normal, but quickly settled down. I let the engine run about 10 min, shifted the transmission in all drive positions, A/C on, and no stumbling, nor hickup or rise in idle. So you wonder what was the fix? Until a new pcv hose arrives, a little dab of superglue on the "t" fitting lines. Once the lines sealed to the fitting, all, for now is well. I have started and run the engine four times, with great success. Will let you know if anything changes.

Tiny
Dlvaughn1963
Oct 4, 2012.
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