low idle when braking hard

[gtyates]

I have an unusal issue. I have a '97 with the OHV engine. Anyway, under hard braking, my idle drops to around 500. As soon as I get stopped, it rises back to 750. I recently replaced the IAC thinking this was it, then the CEL popped on and I replaced the EGR sensor and cleared the code. Problem still persists, but only under hard braking. Brake pads on the front are near the end of their life, but not too thin for specs. Any ideas?

Other history, I have replaced the MAF Sensor about 2 years ago and the upper intake gaskets about 1 year ago. Truck has 130k. No CEL is on. Brake fluid level is normal.

 

[Willard]

I don't see that as a problem. Mine idle's around 500 normally anyways.

I've seen a lot of vehicles whose idle drops a little bit during hard braking. I think it may be the computer taking fuel away to assist with the braking and to up the vacuum that is much required for the brake booster.

 

[gtyates]

Thanks for reply. Part of me thinks this issue is purely related to this "hard braking", but I am still not totally sure. The truck also idles a little rough. Not bad, but not totally smooth either. I notice a slight fluctuation on the tach as well. I can almost feel a slight "shudder" when idling, but again, not enough to call a shake. My wife's '04 with the SOHC idles much smoother. Of course, with no CEL figuring out an issue is a challenge.

 

[Willard]

If the idle isn't steady, that could make it worse too. How long has it been since a good tune-up? Plugs, wires, PCV valve, clean IAC and MAF. I bet these will make a world of difference.

 

[gtyates]

Thanks again. Further history on my vehicle. MAF was replaced about 2 years ago. IAC was replaced about 3 weeks ago. CEL did come on about the same time, indicating high egr flow. Replaced the EGR sensor (which had some moisture in it), cleared the code, and CEL has not been on since. Also replaced the heater control valve about a month ago, which had developed a leak. Related to this, the leak from the heater control valve did drip down onto one of my spark plugs. Could this plug maybe be fouled? I see the residue on the spark plug wire, though not on the plug itself. Plugs were replaced about a year ago at the same time the upper intake gasket was replaced. I just wish the CEL would illuminate again giving me code to work with. Very unusual problem. And, this issue is more prevelant after truck is completely warmed up. Seems to run better when cold, though the idle is still not as smooth as it should be. I have no fluid leaks, oil level is great, brake fluid level is great, coolant level is great, everything seems to be as it should except for this idle issue. Truck has 130k and has been very well maintained.

 

[gtyates]

Follow-up

I think I have found the problem. I have been thinking it was a vacuum related issue all along, and I discovered it is. The vacuum hose that runs from the air intake to the oil filler tube had come loose at the air intake. I think it is possibel when the heater valve was replaced they accidentally pulled it loose. So basically, unmetered air was getting through past the MAF but before the IAC. After placing the hose back, I have driven the truck for quite a while this evening, purposely going thourgh several neighborhoods where I had to make many stops at stop signs. Engine never stumbled, no matter how hard I hit the brakes. So, I feel really good that I have resolved this strange problem!! Now, to work on the rough idle (which is a little smoother since reconnecting the hose) If this does not fix it, I will post further to maybe help someone else out.