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.

RE: 3.5 High Rev Problem
 

More info on the idle problems with the 3.5. Compression is good in all cylinders. Plugged in a code reader and it indicated a bad idle air sensor. Could that cause a lack of vacuum from the upper plenum?

RE: 3.5 High Rev Problem
 

It is what sends the signal to the PCM to open the idle air valve. Most definitely could be the cause of the entire issue.

FYI, you don't have vacuum because your engine is running at high RPM. Vacuum is diminished as RPM increases. (700 rpm = Maximum vacuum, 4000 rpm = Almost no vacuum) The idle air valve being open because the sensor told it to will cause this. It also happens when you put your foot on the throttle. The motor and the vacuum (mechanical parts, not sensors) don't know the difference. While we control most of these parts with computers and electronics, the parts themselves must still conform to the laws of physics.

Replace the sensor, AND check the idle air valve. It could be stuck open.

RE: 3.5 High Rev Problem
 

What exactly was that code? I don't think there is such a thing as an idle air sensor. There is an idle air control motor that gets its instructions from the engine computer. The computer uses engine coolant temperature, gear position, battery voltage, ambient/battery temperature, vehicle speed, throttle position and manifold absolute pressure to determine aproper setting for the idle air control motor. The idle air control motor is located on the throttle body, just under the intake/throttle area.
The sensors that control this generally have self-checking routines, and will generate their own error codes, so your problem may be isolated to the IAC (idle air control motor). Make sure it's plugged in properly. Consider replacing it.

RE: 3.5 High Rev Problem
 

Just replaced the idle air control valve with no better results. I am leaning towards a big vacuum leak somewhere. Do the vacuum resevoirs go bad? I'm getting frustrated!!!!!!!!!!!!!!!!!

RE: 3.5 High Rev Problem
 

Was that code reader you applied capable of reading Chrysler codes? You should have gotten a code for "Target idle not reached" - P1294 if the engine would not achieve a proper idle speed.
If you have a significant vacuum leak, you should have gotten a P1299 code.

There is a vacuum reservoir, it operates off the vacuum going to the brake booster and provides vacuum for the speed control. The check valve for the wholevacuum system is located at the booster. You might try disconnecting and plugging that line. If that solves your problem, then the trouble is going to be found back up somewhere behind the plugged location.
The reservoir itself has no check valve. It's located in front of the battery behind the front bumper fascia, so it's not real easy to get to.
Also check the EGR line as that's another possible source of a vacuum leak.

Keep us posted. We share your frustration and will share your joy when this is finally fixed.

check your pcv