3.5 High Rev Problem
 

Today I removed the upper plenum(have done before with no troubles!) and replaced a heater hose. After putting everything back together I started the car and it immediately revved to 4000 rpm. All vacuum connections seem to be o.k. but the power brakes feel as though they have no boost. I'm stymied! Anone out there able to point me in the right direction? Thanks in advance.

RE: 3.5 High Rev Problem
 

Obviously a vacuum leak. Probably a broken/bad/disconnected hose, or maybe even the gasket on the intake itself.


If a visual inspection doesn't help (could be underneath), spraying intake cleaner at all the intake seams might pinpoint the problem area. If the idle speed changes where you spray the cleaner, that's it.

Good Luck

RE: 3.5 High Rev Problem
 

A stuck idle air valve can do it, a stuck EGR valve can do it, and you may have messed with both of them while removing the plenum. Also debris in the throttle plate or a kinked throttle cable.

Your power brakes are not likely to work well at 4000 rpm because there is no vacuum to assist them regardless of why.

Spray carb cleaner around all joints and hoses. You'll hear the wet sucking sound when it finds a vacuum leak. You'll also notice a drop in rpm at the same time.

RE: 3.5 High Rev Problem
 

Some more info on my high-revving 3.5. When I put it into gear or some other function that puts a load on the motor the idle drops down to 700-800 r.p.m.s, but it backfires through the intake and exhaust. Also, still no brake boost at any r.p.m..

RE: 3.5 High Rev Problem
 

Throttle position sensor out of whack? Maybe MAP sensor? Is it throwing any codes?
Some of these sensors have identical connectors. Did you mix any of them up?

RE: 3.5 High Rev Problem
 

Would any damage be done if the car was started when the upper plenum was apart from the lower plenum? If so, what? Thanks

RE: 3.5 High Rev Problem
 

I can't imagine it would start like that. With no throttling possible, no TPS, no MAF, no MAP sensors?
I wouldn't think it would hurt much unless you sucked a gasket into the lower intake manifold. That would not be good.

Go back and double check all the intake related sensors, Throttle position sensor (TPS), Mass air flow sensor (MAF), Manifold absolute pressure(MAP), Intake air temperature (IAT). Make sure they are in place and wired up correctly.

Buy or borrow a code reader and see if any codes are reported by the computer. Address any problems found.

RE: 3.5 High Rev Problem
 

Thanks, dcotter. All of the sensors were still hooked up. The upper throttle body was loosened enough so I could easily get to the heater hose. I will try all suggestions you offered. Does the fact that sensors, throttle, ect. were still attached change any of your suggestions about diagnosing the troubles?" Would internal engine problems show up on a code reader? Thanks again!

RE: 3.5 High Rev Problem
 

I'm thinking maybe one of the gaskets between the upper and lower intake manifolds may have gotten misaligned as a result of the start with the manifolds separated. I realize that means pulling it off again but I would definitely want to rule that out. If you didn't disconnect any of the sensors, then it's probably very unlikely that they got mixed up or miswired.

RE: 3.5 High Rev Problem
 

It is obvious that you have lost control over your airflow, and possibly your throttle. Check your throttle cable for kinks or misalignment along with everything that holds vacuum.