Throttle Body Service

We can repair most throttle body problems. There are several problems that can develop with the complex ZR-1 throttle body. It has three throttle plates and two stages of operation. All of the problems manifest themselves with one symptom. A high idle speed. The factory specification for the LT5 idle speed is 650 rpm. When using an engine data scanner the normal cold engine IAC step count is about 60. The normal hot engine IAC count is 10. When the IAC goes to zero the idle speed can not be reduced by the engine control system. Then the engine idles at more than 650 rpm.

High idle speed is most commonly caused by a vacuum leak. The throttle body can cause a high idle if a throttle plate binds with a bore. If the high idle is intermittent or goes away when the accelerator pedal is kicked, the high idle is caused by the throttle body. If a light tap fixes the idle, the problem is with the primary throttle. If a hard kick to 25% throttle or more helps, the problem is with the secondary throttles.

The secondary throttle shaft in the LT5 throttle body is normally relatively loose with .004" clearance. I believe that the designers did this to avoid binding of the shaft due to small misalignments. How the operation of the throttle body is effected by the fit of the shaft is dependant with the fit of the throttle plates to the bores. We can repair throttle bodies with up to .006" of shaft clearance by refitting the throttle plates to the bores. As the throttle body wears, additional friction can develop in the mechanism. Also with use, the three return springs can lose a portion of their tension. We have remedies for those problems. We set the throttle position sensor to the ideal .53 volt setting. The OE socket drive throttle stop screw is upgraded to an 8 mm hex head screw that is easy to adjust when the throttle body is on the engine.

Refinishing the top surface is not included. It is available at additional cost.

The cost for the repair service is $295.

DAG and the LT5 Throttle Body

As far as I know GM has never documented any information about the use of Dag as a sealing and anti-friction method on the LT5 throttle body.

Dag is a general purpose material that is graphite based and is applied like paint. It's used in various industrial applications that need properties like a thick material buildup, low friction and high temperature range. DAG is a trademark of the Verick, Acheson Colloids company.

This is a '94 throttle body with 7000 miles. The Dag coating can be seen on the bores and on the edges of the secondary throttle plates. It is not used on the primary throttle.

  Click on the pictures to enlarge them.

This is a '92 throttle body with 6,600 miles. The throttle plates have been removed for service.

It is easy to mistakenly identify the Dag as dirt. Dirt builds up on the Dag in the normal operation of the engine. These throttle bodies were cleaned so as to preserve the Dag. They were simply gently wiped with a soft dry cloth. The use of a solvent will remove the Dag.

The Dag was used to improve the seal of the secondary throttles. There are problems with the Dag coating. It wears off or chips off quite easily. It will come off if it is wiped hard. It will come off very easily if a solvent is used. It's easy for an uniformed maintenance person to remove it unintentionally. At a glance it looks like dirt.

Loss of the Dag coating can cause excess air leakage past the throttle plates and loss of idle speed control range for the engine control system.

I recommend that the LT5 throttle body bores should never be cleaned. In my experience a normally dirty throttle bore causes no problem. In fact a little oily grime around the throttle plates helps seal the throttles to minimize air leakage. It also lubricates the edges of the plates to avoid sticking. If the bore is extremely dirty or you just can't resist the urge to clean it, do it gently with a soft dry cloth. Then apply a light coating of engine oil to the throttle plates.

I have tried to reseal throttle plates with little success. The problem has been with poor adhesion to the bore with the sealing material.

When the Dag is no longer effective, I improve the fit of the throttle plates to the bores with a burnishing process.