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1990-1995 ZR-1 Secondary Port Vacuum Diagnosis


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Staff member
Sep 16, 2000
New Hampshire
1990 Corvette ZR-1

LT5 Secondary Port Operation & Vacuum Diagnosis​

Before getting into diagnosing the LT5 engine's secondary port vacuum system, I thought it would be a good idea to take this opportunity to clear up a few popular misconceptions about the LT5 engine and establish a basis for understanding what the engine is and how it operates.

The heart of the 1990-1995 ZR-1 Corvette is the LT5 engine; one of the most technologically advanced engines of its time. A lot of misconception seems to exist about the history and operation of the LT5 engine.

Rather than go into great detail here about the engine, a few facts should be addressed up front:

  1. The LT5 engine is an all aluminum, double overhead cam design with 32 valves, and 16 fuel injectors (8 primary, 8 secondary).
  2. Power output was as follows:
    • 1990 - 1992: HP: 375 hp @ 6,000 rpm, Torque: 370 ft-lbs @ 4,800 rpm
    • 1993 - 1995: HP: 405 hp @ 5,800 rpm, Torque: 385 ft lbs @ 4,800 rpm
  3. The LT5 motor was designed and engineered by both GM Powertrain in Detroit, MI *and* Lotus Engineering in Hethel, England. It was not solely designed by Lotus Engineering. It should be noted that at the time, Lotus was owned by General Motors.
  4. The motor was built by Mercury Marine in Stillwater, Oklahoma. Mercury Marine was chosen by GM to build the LT5 due to their expertise in building aluminum outboard motors and their ability to manufacture engines on a small scale.

Valet Key ("Power Key")

Since there is a lot of misunderstanding about what the purpose of the Valet Key is and how it works, I thought I would include the following from page 6E3-C2-2 of the 1990 Corvette Service Manual:

"The driver can select two engine power modes, "FULL" or "NORMAL" power, with a a console mounted key switch. This switch is wired into the ECM, and allows the driver to determine the engine output by controlling secondary port throttle valves, secondary injectors and secondary fuel pump operation. When the key switch is in the "FULL" power position, the "Full Engine Power" light, located in the driver information center, is illuminated."

"In the "NORMAL" power position, the ECM does not allow the secondary port throttle valves to be opened, or allow the secondary injectors to be turned "ON." In the "FULL" power position, the ECM enables secondary operation only if no trouble codes are stored and other criteria are met..."

It should be noted that with the key in the "NORMAL" power position, the engine is limited to approximately 210 horsepower.

The following information below comes from Tim Holland, a Lotus Engineer who started at Lotus in March of 1985 and began work on the LT5 project in July of 1985. Among his other responsibilities for the project, he served as the mechanical/calibration engineer as well as the emissions engineer responsible for meeting EPA requirements.

Secondary Port Throttle Operation

In order for the secondary port throttle valves to open, the following parameters must be met:


When these parameters are met, the ECM will signal the ports to open and wait for .5 seconds before turning on the secondary injectors. As the secondary injectors share the same drivers as that of the primaries, the pulse width signals are halved when the secondaries operate.

As shown in the diagrams (below), there are three different styles of driving which will operate the port throttles:

  1. Ref. line A-B. Constant throttle position with a rising rpm. At the point the arrow corsses into the 'Port Throttle Opening Region' (top left shaded area) the port throttles open and the secondary injectors start firing.
  2. Ref. line C-D. Constant rpm with increasing throttle position - logic as above.
  3. Combination of the above: i.e. increasing throttle and rpm - logic as above.

Secondary Port Throttle Vacuum Leak Diagnosis​

One of the easiest ways to determine if you have a secondary port vacuum leak is to simply turn the ignition key to the on position but do not start the car. You should hear the Secondary Port Throttle Vacuum Pump turn on. If you don't hear the pump turn on, it's possible that it could be burned out.

The pump is located in front of the coolant reserve tank right underneath the headlight bucket on the passenger side of the engine compartment (see Figure 1 below). The pump should run for 5-10 seconds to prime the system and then turn off.

Figure 1 - Secondary Port Vacuum Pump - Part #10098474 (Also check Part #10098493)


Unfortunately the quality of this photo is not as good as I'd like but good enough for this example. This is the secondary port vacuum pump located under the passenger side headlight bucket

So what does this pump do? There is a vacuum reservoir located under the plenum. If the vacuum level in the reservoir drops below 41 kPa (12" Hg) while the secondary port throttle valves are open, the pump is turned "on" by a vacuum switch to supplement engine vacuum.

If the pump cycles on and off every few seconds or it runs continuously, chances are, you have a vacuum leak somewhere in the system.

To begin, take a ride to your local Sears Automotive Center and purchase a hand held vacuum pump as shown below in Figure 2. This pump is extremely invaluable and will help you to locate the source of the vacuum leak. The pump comes with all necessary hose adapters.

Figure 2 - Handheld Vacuum Pump


Figure 3 below shows the vacuum connection between the line going to the pump (right - see pump below) and the left hand vacuum system going underneath the plenum (left).

Figure 3 - Vacuum Connection


Disconnect the hose at this connection. At this point, with the engine key in the "On" position, the pump should turn on and run constantly. (Another way of verifying that the pump is operating correctly) Connect your hand held vacuum pump to the line going under the plenum and bring the system up to 10-12 Hg. If the system does not have a leak, the gauge needle will stay at that level. If there is a leak, it will slowly move back to zero. If there is no leak under the plenum, repeat the procedure at the other end of the hose going to the pump to determine if there is a leak on that side of the system.

If you have a leak under the plenum, it's time to remove the plenum from the top of the engine. To do this, follow the procedure outlined in your service manual for plenum removal. You should always have a service manual for your model year prior to conducting any diagnostic tests or repairs. If you don't have a service manual for your car, you can purchase one from companies such as MidAmerica, Ecklers or Helm.

To put everything in perspective, below is an LT5 engine with the plenum on (Figure 4 below). With the plenum off, it's a much different picture than what you see here!

Figure 4 - LT5 Engine Prior to Plenum Removal


Figure 5 - Secondary Port Vacuum Hose Assembly


In this photo, you can see the entire secondary port vacuum hose assembly (Part #10146221). At the lower right you can see the connector that is pointed to in the picture above. The upper left hand corner connects to another connector under the ECM.

Below is a picture with the plenum removed (Figure 6A).

Figure 6A - Plenum Removed


1. Primary Port
2. Secondary Port
3. Secondary Port Control Solenoid
4. Secondary Port Vacuum Actuator (Part #1996743) (just to the right, you can see the metal arm and attachment going to the injector housing
5. Fuel Rail Assembly with injectors inserted into the Injector Housing

Figure 6B


Figure 6B is a closeup view of a primary and secondary port. The primary port is on the right and the secondary port is on the left. You can see the butterfly valve inside the secondary port.

Below is a picture of the LT5 engine throttle body.

Figure 7- LT5 Throttle Body


In the center of the throttle body, at the bottom, you see the primary throttle valve which is 22 mm (0.87 in.) in diameter. Above are the two larger secondary valves which are 59 mm (2.32 in.). The secondary throttle valves begin opening when the primary valve is 80% open and are completely open at full throttle.

From page 2-9, Corvette LT5 Fuel and Emissions, Course No. 16009.14-1:

"The primary throttle valve is operational when the vehicle speeds are less than approximately 113 km/h (70 mph) (level terrain cruise). The secondary throttle valves are opened at approximately 80 degrees of primary throttle opening."

Figure 8 - Top View, Plenum Removed


1. Secondary Port Vacuum Reservoir (Part #10067563)
2. Crankcase Ventilation Cover
3. Secondary Port Vacuum Actuators (Part #1996743)
4. Ignition Coil Packs (starter is underneath them)
5. Vacuum connector which plugs into the Secondary Port Vacuum Solenoid
6. Crankcase Breather Hoses (Part #s 10110908, 10110907)

In this picture you can see the top of the Crankcase Ventilation Cover. For those of you with 1990 ZR-1s, this is a common area of oil seepage. The cover bolts were not torqued properly from the factory and can loosen up over time. If you notice the top of your engine V is damp with oil, chances are, this is the source. It's a good idea to replace the Crankcase Breather Gasket underneath (Part #10168682) and re-torque the bolts.

Figure 9


1. Vacuum Solenoid with conical air filter (Part #1997222), Conical Air Filter Part #: 10159525
2. Injector Housing
3. Fuel Rail

Figure 9 is a different angle taken from the driver's side of the engine looking down at the vacuum solenoid.

Figure 10


From LT5 Fuel and Emissions, a GM STG Training Course Book, pages 2-12 and 2-13:

Secondary Port Control Solenoid

This solenoid is used by the ECM to control the application of vacuum to the secondary port throttle valve actuators. The solenoid is normally closed and is opened ("ON") when the ECM provides the ground path. The solenoid is located under the intake plenum at the front of the cylinder block valley, near the left hand side of the vacuum reservoir. The sensor is connected to the engine wiring harness by a two terminal blue connector. (Refer to Figure 2-15)."

Figure 11


Secondary Port Throttle Valves, Actuator and Linkage

"The secondary port throttle valves are used to provide additional control of airflow to the cylinders. The valves are vacuum operated, and controlled by an ECM actuated solenoid, based on input signals from the Throttle Position Sensor (TPS), engine RPM, engine oil temperature sensor and Manifold Absolute Pressure (MAP) sensor. Vacuum is supplied through the vacuum reservoir and vacuum pump. (Refer to Figure 2-14)."

"In operation, the secondary port throttle valves are closed during low engine speeds and loads, allowing no air path to each cylinder. Additional air is allowed to enter the cylinders at higher RPM when the following criteria are met:"

"When the TPS indicates to the ECM that the driver has pushed down on the accelerator pedal far enough, and the oil temperature sensor indicates engine oil temperature is within an acceptable range, the ECM will turn "ON" the secondary port throttle actuators through the secondary port throttle control solenoid allowing engine vacuum to be applied to the actuators to open the valves. At the same time, the secondary injectors are turned "ON" to provide additional fuel flow to maintain the correct air/fuel ratio for maximum power (12:1)."

"The secondary port throttle valves are located in the right and left hand cylinder head (four plates per head). The actuators are mounted to brackets attached near the center of each cylinder head. A common linkage incorporates each set of four throttle valves together to the actuators which will open all throttle valves in unison (Refer to Figure 2-14)."

At this point, if you trace the various connections of the vacuum lines, you'll see that they go to the secondary port throttle valve actuators, vacuum solenoid and vacuum reservoir. These are the three main areas for vacuum leaks to occur due to mechanical failure of these parts.

1. Test the Secondary Port Throttle Vacuum actuators by disconnecting the vacuum line, connecting your vacuum pump and applying enough vacuum until you see the secondary port throttle valve plates open. Once open, they should stay open until you release the vacuum from the pump. If they slowly start to close, chances are the secondary port throttle valve actuator needs to be replaced. Conduct this procedure for both actuators.

2. Test the Vacuum Reservoir by connecting your vacuum pump to one inlet and blocking the other. Apply 10-12 Hg of vacuum. If the gauge starts to decrease, replace the vacuum reservoir.

3. Test the vacuum solenoid which is in front of the left hand side of the vacuum reservoir. Make sure all connections but one to the solenoid are blocked and connect your vacuum pump to the free connection. Apply a vacuum as described in step two. If the gauge decreases and vacuum is lost, the solenoid needs to be replaced.

Case in Point​

Recently, I noticed that my secondary port vacuum pump was cycling on and off when the ignition key was turned on. I started off by confirming that a vacuum leak existed under the plenum. After removing the plenum I checked the vacuum at many of the connections and found that the vacuum solenoid was not holding a vacuum. Upon replacement, the vacuum pump primed the system, turned off and did not continue cycling on and off as before.

As you can see below, I had a couple "visitors" who came to my garage wondering why there were tools and a plenum strewn all over the garage floor:



Keep in mind that if you run vacuum checks on the three main areas I outlined above and they all pass the vacuum check, it is possible that you have a leak somewhere else in the vacuum lines. The vacuum line system consists of several hoses and check valves that are routed throughout the "V" under the plenum.

Make sure that before you put the plenum back on the engine, all vacuum lines and electrical connectors are reconnected! This is very important because all it takes is one failed connection and you will be pulling the plenum back off to determine why the secondaries don't work at all.

As you can see, the most difficult part of diagnosing and repairing most secondary port vacuum leaks is the removal of the plenum. Once you get the plenum off, you have easy access to most of the key components that control secondary port vacuum operation. Good luck!
Last edited:
Great posting Rob. I did'nt actually do the work but I had a mechanic friend of mine do the same work. I remeber this vacuum lines being combination of rubber coupling and hard plastic tubes. A lot of times, leak occurs due to separation of rubber fittings from the plastic tube. I am thinking about using factory rubber fittings and copper tubes bent to mimic factory plastic tubes (known to crack due to heat) and using weather strip glue to seal copper tube to the rubber fittings. Do you think this is doable? Also, when replacing these vacuum lines, smallest plastic wire ties is an additional insurance against having to do this again.
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I can't say for sure. I'm not enough of an engineer/mechanic to say that it would or wouldn't work. Let's see if anyone else can contribute to this.
Excellent Job on the LT5 Secondary Port Operation & Vacuum Diagnosis, Rob ;worship ;worship

In regards to the possible leaks from the plastic tubes where they attach to the rubber fittings, I've had good luck using a small drop of "super glue" applied to the side of the plastic line before pushing it into the rubber fittings.
Very informative article Rob. Nice job.

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Tonight I was fumbling through all my original parts that I've replaced on the ZR-1 and came across the original secondary port vacuum pump. The problem with these pumps is that they are not sealed very well against the elements. Over time moisture can accumulate and degrade the components inside. Below are two pictures of the inside of a secondary port vacuum pump. You can see the copper oxidation on the right hand side (bluish-green powder)

<div align="center"><img src="/tech/c4/zr1/images/pump1.jpg" width="549" height-"588" border="2" alt="Internal view of the secondary port vacuum pump" /></div>

<div align="center"><img src="/tech/c4/zr1/images/pump2.jpg" width="550" height-"413" border="2" alt="Internal view of the secondary port vacuum pump" /></div>
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Since this was asked in the ZR-1 Forum, I'll supply the same information here to be added on to this column article:

Secondary Port Actuator Valves (1996743)
List: $42.00 ea.
Net: $31.50 ea.
Purchased: 5/26/99

Secondary Port Vacuum Reservoir (10067563)
List: $13.00
Net: $10.35
Purchased: 5/26/99

Secondary Port Vacuum Solenoid (1997222)
List: $28.03 - 6/7/99
List: $39.33 - 5/29/02

Net: $19.62 - 6/7/99
Net: $31.46 - 5/29/02

Secondary Port Vacuum Pump (10098474)
- Given to me - did not pay for.

Secondary Port Vacuum Hose Assembly (10146221)
List: $15.60
Net: $11.70
Purchased: 5/17/99

Throttle Body Gasket: (10067759)
List: $2.20
Net: $1.65
Purchased: 5/18/99

Throttle Body Gasket: (10168680)
List: $20.65
Net: $15.49
Purchased: 5/18/99

Crankcase Breather Gasket: (10168682)
List: $23.95
Net: $17.96
Purchased: 5/17/99

Crankcase Breather Hose: (10110908)
List: $9.10
Net: $6.83
Purchased: 5/24/99

Crankcase Breather Hose: (10110907)
List: $9.10
Net: $6.83
Purchased: 5/24/99

Source of all parts: Chevrolet Dealership

Great and very big pictures

Good help when you rebuild engine / vacuum system !

MORE OF these items please ......

'91 geiger corvette
Holland:v :v
Very, Very, technically astute & informative post!

Many Thanx Rob!
You're welcome. Glad the article is helpful. :)
Hesitation When Engine Hot

Rob, GREAT article about the Secondary Port Vacuum on the LT5. It's posible that could be my problem. I have a 90 with 40,000 miles on it. Runs much smoother from first start up. After it's good and hot the hesitation begins right at the time the secondaries kick in. Changed the plugs and have updated to a new PROM. Still have what I feel is an extended hesitation ( not the .5 seconds ) which is normal. Before I start ripping this apart, is there the possibility that when the engine is good and hot after 10 minutes this would cause the vacuum lines to leak. Thanks, Jeff
JBEGOR said:
Rob, GREAT article about the Secondary Port Vacuum on the LT5. It's posible that could be my problem. I have a 90 with 40,000 miles on it. Runs much smoother from first start up. After it's good and hot the hesitation begins right at the time the secondaries kick in. Changed the plugs and have updated to a new PROM. Still have what I feel is an extended hesitation ( not the .5 seconds ) which is normal. Before I start ripping this apart, is there the possibility that when the engine is good and hot after 10 minutes this would cause the vacuum lines to leak. Thanks, Jeff
Hi Jeff,

It's hard to say. I suggest running a vacuum check as indicated in the article when the engine is both cold and hot and compare the differences. If you're losing vacuum when the engine is hot, then I would start pulling the plenum off and looking for the leak.
ZR1 Vacuum Leak

Hi Rob. Thanks for your write up. I have a problem. My ZR1 has a vacuum leak! I called the local Chev dealer to get a new vacuum hose assemble. part #10145221, They told me they are no longer avallable from GM. Do you or anyone else know where I can find a new one? Thank You. Eli.
ROUTE66 said:
Hi Rob. Thanks for your write up. I have a problem. My ZR1 has a vacuum leak! I called the local Chev dealer to get a new vacuum hose assemble. part #10145221, They told me they are no longer avallable from GM. Do you or anyone else know where I can find a new one? Thank You. Eli.

What does that part number go to?
ZR1 Vacuum Lines

Rob said:

What does that part number go to?
Hi Rob. Thanks for responding. It's called a Secondary port vacuum hose assembly. I got that part # from your post on[Secondary port throttle vacuum leak diagnosis] In one of your pictures it shows a set of vacuum lines & down below the picture is that part#. Im the original owner of a low milage 93. The other day when I went to start it it backfired & blew off a vacuum line at the MAP sensor. I put it back on IT runs ok except sometimes it has a slite hickup when giveing it gas after up shifting. I also found a small amount of oil in the vacuum connection at the MAP sensor. So I thought I would replace the vacuum lines. Any help would be very much appreciated. Thanks.
Rob, great write up!! I'm a new member to the CAC and also a new member to the Corvette world. Being new I have been going through old post and really any info I can absorb to better understand the zr1. My z runs great and the previuos owner had the plenum off when he repaced the starter and checked for any vaccum leaks when he put everything back together. He said eveything underneath looked new, he said the dealer he bought the car from had just did a full tune up. I doubt they did. The wires and the plugs look original to me. My question is after reading this post I went out and turned the ing to the on position heard the pump come on and go off, but it came back on in about 15 seconds went off and then came back on in 15 seconds and just kept doing the same cycle. Is this normal or is this what you ment by cycling on and off? I will buy a vaccum tester this week and run a test that you outlined in this post. I want to thankyou again for so many of yours and everyone else on the forum that has the experiance with these awesome cars and shares that info with us newbeies.:thumb
1990 zr1

My vacume pump cycles on and off but has no affect on the performance of the car. As long as the pump maintains vacume why go to the trouble of replacing lines if the leak is not that bad?

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