I have a small 90deg rubber boot on the top of my engine on my 2003 trailblazer that has dry rotted and is causing my engine to have a rough low idol. I went to our local auto parts store and they did not know what this part was. Can you help?

The brake booster can do this. It is the big black circular thing on the drivers side, at the fire wall. A small hole in the diaphragm. Hard to find and diagnose. These aren\'t cheap. Get one at a junk yard... New ones are $200 to $500. Small vacuum leaks will always cause an acceleration. big ones cause a misfire or really rough idle.

check fuel pressure regulator for fuel in vacuum line ,check fuel filter , and alot of those models needed a top engine clean where u drip top engine cleaner through the air tube to clean valves .
we have done this action quite a few times with good results where i work
for some reason these 4.2 ltr engines seam to gum up on valves alot

Is the check engine light on? If so have you checked for codes? Do you have a P0171 and P0174 codes?

If so, and your symptom supports this, you may have a vacuum leak. This could be anything from bad plenum gaskets to a rotted out rubber elbow. Normally I would suggest using carb spray to find the vacuum leak but carb spray is flammable and since the coils are on the top side of the engine you need to be really careful not to get it near the coils as the engine could catch fire.

What I would suggest is having the engine smoke tested. This uses a machine that pressurizes the intake system with a low pressure smoke. the smoke will come out of wherever the leak is.

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Find the PCV tube and look at the rubber elbow where it attaches to the PCV valve, it tends to rot and leak vacuum causing your problem.

For the drivers side, reach down by your alternator and over behind the headlight. You'll feel a 3" round rubber boot. Slip this off and now you can remove the light bulb.

Passengers- Remove the top of the air cleaner, at that point you'll have complete access to the rubber boot and bulb.

172 is caused by a rich fuel mixture on the right side. It may be caused by a fuel injector not closing fully. Dirt particles can do this. Adding Gum out may help. The other engine codes, 306 and 340 is for missfire which the cam sensor is at fault

it sounds like you might want to scan your vehicle for sensors evaluation to determine what causing your rough idle if you have engine light on

These hoses are starting to dry rot which is causing the small "holes". They're easy enough to replace so go ahead and swap them out.

pcv cracked? The spark plugs are installed into the top of the head.
Replace cracked or dry rotted hose or the gromit for the crank case pressure valve, this should return through a line to the intake. Good luck

I had a similar problem, check engine light would come on and all pointed to the mass air flow sensors and oxygen sensors. I was searching the internet to locate those little suckers when I ran across someone that said to check the little L-shaped rubber boot at the top of the engine near the firewall. I said what the heck, I went out and finally found it, I put my thumb on the hose and it was caved in. I put some pressure on the hose and it snapped in two. I drove my other vehicle to Autozone and they had a L-Shaped rubber hose with the same fit but bigger, so I cut it to size and I have not had one problem. I did replace another hose that looked dry rotted and checked all my vacuum lines to ensure that they were attached. This was over 3 months ago, I am not burning oil like I was and I am getting better gas mileage and the ride is superb. $7 for the hose. To replace all of those sensors would have cost me over $400. This may not be your problem but it would not hurt to check. Good Luck!