Vacuum Issue?

I installed a vacuum gauge in my dash and now I noticed an issue. While crusing at a constant speed 40 - 60 mph or more vacuum remains at about 12 BUT goes up to 18 for a second then back to 12. This happens about every 3 or 4 seconds. RPM doesn't change and you can't feel it while driving. I've tried 2 different gauges with the same results. Is this normal? Could it be a pulse of some kind like for EGR or something?

My first step if concerned would be to isolate all the vacuum 'consumers' on the engine, so the only thing externally connected was the gauge.
If the vacuum continued to pulsate every 3-4 seconds, you would then know that it was something internal or influencing the engine and hot it runs. If it ceases, then the pulsation is a leak or external vacuum 'user' that is opening/closing its 'port'.
Experiment a bit and let the world know...
tom

Those are the speeds where I would expect to see EGR use and evap purge happening. Both could affect manifold vacuum.

When was PCV Valve last changed?

I would change it on spec, if its older than 2 years

I wouldn't think EGR because of the "no change" in drive-ability, it is a short time but cutting off exhaust gas flow should create a blip you would feel?

EVAP solenoid could also be the culprit, but might be normal operation of the system, but I wouldn't think so.

You could cap off any of these systems and see if fluctuations stopped, you would get EGR or EVAP code but it would go away once vacuum was restored.
EVAP might take a few drive cycles to reset

Where?

Thanks for all the great suggestions. Disconnected and plugged EGR vacuume line and issue stopped. Took valve off and checked it with vacuume pump. Held vac and opened valve on bench. so EGR OK? What controlls the vacuume to the egr? Could problem be the module attached to firewall that has the plug and two rubber tubes coming off the metal tube going to EGR? I don't feel like spending the money on the module if it is not the problem.
TIA

The EGR system is a monitored system, and a Emission system so you would absolutely get a CEL and code if there was a problem there.
If the vacuum pulse stopped with EGR capped then my guess would be that it is "normal" or at least not outside of computers range for proper operation.

EGR system has 3 parts, 4 if you include the computer(PCM)
EGR Valve
EGR Modulator/solenoid
and
DPFE sensor, part you described

Computer monitors engine load, under load the cylinder temps go up, which will cause higher NOX emissions, toxic gas.
So computer will pulse 12volts to EGR Solenoid, this causes vacuum to be applied to the EGR Valve, and this lets some exhaust gas flow into the intake.
The HOT exhaust gases mixed with the air/fuel mix cause the mix to burn slower in the cylinders, so they burn cooler, and less NOX.

DPFE sensor is the feedback to the computer on how far open the EGR Valve is, how much exhaust gas is flowing into the intake.
One hose on the DPFE sensor is closer to the EGR valve than the other so the closer hose will have a lower pressure as EGR Valve opens.
This pressure difference tells the computer how much exhaust is flowing into the intake.
DPFE = differential pressure feedback

The computer might be programmed to stop pulsing EGR solenoid to test if DPFE sensor is working, when EGR solenoid closes vacuum in intake goes up and DPFE should be 0 because EGR Valve is closed.
Then after 3 or 4 seconds computer activates EGR solenoid again, vacuum goes down, adding exhaust gases back to intake to stop the NOX production

It kind of makes sense since there really is no other way to test/monitor the system except when it is being used


One heads up on DPFE sensors, now that you know where it is, exhaust has water vapor in it, a by product of burning Hydro carbons(H) with air(Oxygen) is H2O(water), which is why exhaust systems rust from the inside out,
The hoses on the DPFE sensor get water vapor inside, and so do DPFE chambers
This can cause bad pressure readings but it also causes DPFE sensor to get corroded inside.
Not a bad idea to make sure hoses are clear once a year, do not reverse them, lol.

what adsm is saying is it's most likely not an issue, as long as you are not seeing a power issue or anything, ignore it... More vacuum (bigger number in in/Hg) means less load on the engine, when the computer is adding in some EGR the engine load is going to increase a little because it is adding inert gas to cool the exhaust valves which exactly explains your description of the vacuum...

If it is related to the EGR I would not worry about it.

The EGR is not a huge vacuum consumer, and normal operation will probably cause a momentary blip on the vacuum gauge.

Egr!

My code scanner gave me a code 332 " Insufficent EGR flow detected".
So do I just ignore it? Plug the vac line to EGR? or just leave it as is?
The rubber lines going to the sensor are all clear. I don't want to cook my valves or waste fuel, (kinda expensive).

Just wondering...

Thanks all!

With the engine at idle use your vacuum pump to apply vacuum to the EGR valve. If it stalls or tries to replace the DPFE sensor.

The code indicates the pressure differential is not being created. The computer will try again and again, and how often or how long, I cannot say.
In the past, the EGR was applied, above idle speed, at steady state on the highway, never at idle. The vacuum would be fed to the EGR valve, and then 'capped', sitting at the amount of EGR the computer liked. It did not change all the time.
The DPFE has two small tubes to the EGR tube. Between the fittings/tubes welded to the EGR, there is a restriction, like a washer smaller than the o.d., welded into the tube. The tubes have higher pressure on the exhaust pipe side, and lower pressure on the intake side, so there is a 'differential'. The computer knows the differential pressure is caused by the amount of EGR gas flowing. More gas== higher pressure, less gas== lower pressure. It will try to open the EGR valve all the way if it doesn't get the diff pressure it wants to see. I think it is trying over and over and over ... and over.
I think you need a new DPFE. Just my opinion. Or you have a stuck EGR valve, or possibly leaky diaphragm in the EGR. The pros will know.
tom