This is a very simple question; does the camshaft play any part in developing manifold vacuum pressure? If the motor is running an undersized cam, could your manifold pressure be below acceptable values for the EGR?

Peace!!
B2

[This message has been edited by BAM-BAM (edited 05-26-2013).]

By manifold vacuum pressure I assume that you mean just vacuum. While an engine is running an intake manifold does not have positive pressure ... it has a vacuum. Somewhere around 15 lbs is good, but with high performance engines a lower reading is normal. A high lift camshaft typically results in lower vacuum which can affect power brakes. To create a vacuum reserve to operate power brakes GM used a vacuum cannister on, at that time, what was considered a high output V6. I have no experience with EGR operation being affected by low vacuum, but I suspect that it is possible. Perhaps another forum member can provide that input.

quote Originally posted by BAM-BAM: This is a very simple question; does the camshaft play any part in developing manifold vacuum pressure? If the motor is running an undersized cam, could your manifold pressure be below acceptable values for the EGR? Peace!! B2

Generally speaking an undersized camshaft duration wise will likely build more vacuum, low end torque and efficiency. Lift may have a small effect considering the extremes of lift are rarely an attribute in the cam selections used for our motors. Duration on the other is another story, the more you increase duration all else the same the less vacuum you will have at idle, overlap plays a role here also. I would imagine a damaged/worn camshaft will have problems developing good vacuum if the lobes are worn excessively making it difficult for the vacuum effect of the descending piston to reach the manifold area.

Ignition timing will also impact manifold vacuum.

It is mostly overlap that lowers vacuum. Longer duration doesn't for sure indicate more overlap, lobe separation angle has a lot to do with it, so a high lobe separation angle will have more vacuum with the same duration cam than one with a smaller lobe separation angle (LSA). Lower LSA's can somewhat compensate for poor intake design at higher RPM but you will loose on the low end. Also, a lot of overlap has the effect of lowering the dynamic compression ratio of an engine because the valves are open longer and some of the intake ends up going out the exhaust before the valves close. This is why you see a suggested minimum compression ratio on some of the hotter cams. Larry

quote Originally posted by trotterlg: It is mostly overlap that lowers vacuum. Longer duration doesn't for sure indicate more overlap, lobe separation angle has a lot to do with it, so a high lobe separation angle will have more vacuum with the same duration cam than one with a smaller lobe separation angle (LSA). Lower LSA's can somewhat compensate for poor intake design at higher RPM but you will loose on the low end. Also, a lot of overlap has the effect of lowering the dynamic compression ratio of an engine because the valves are open longer and some of the intake ends up going out the exhaust before the valves close. This is why you see a suggested minimum compression ratio on some of the hotter cams. Larry

It pretty much works the same with duration after a point as the valves stay open longer and longer as duration increases allowing more air to escape being trapped on the compression stroke since air escaping through the exhaust valve on compression due to a narrow LSA or air escaping back out the intake as a result of still being open as the piston is moving upwards is still a reduced amount of air being trapped. The counter is higher rpm efficiency as it takes spinning the engine faster to speed the valve events up so that less air can escape the compression stroke. Case in point, if you never close the intake valve, you'll never compress any air no matter how wide or tight the LSA.

You are right of course; my point is that two cams with the same duration can have very different chiastic in the same engine. A smaller LSA will generally have a better top end performance than one with a large LSA. Also, the centerline angle has a bunch to do with how it acts. I think most people just look at duration and lift and figure that is it, but there is way more to how an engine acts with a given duration than that. Larry

Like mentioned above, there are several things that can affect manifold vacuum. The camshaft profile is one of them. Generally speaking, a camshaft with more overlap will have weaker manifold vacuum. But a "small" camshaft, like the stock camshaft for example, should generate strong manifold vacuum.

I'm assuming you're having trouble with your EGR system. The first thing to check would be the vacuum lines. Also, check to see if the EGR tube is cracked. And finally, the EGR solenoid itself is a common failure point.

Blacktree, the EGR is the issue and everything else is working as designed. When I built the motor I had an issue (or so I thought) with the cam I chose and replaced it with one slightly milder than the stock 3.4 cam. I have had this EGR issue for over two years and have replaced every component on the EGR system with new parts; Stainless vac lines, new solenoid, new EGR valve and new tube. I have taken the voltage readings and tried every redneck trick that people have thrown at me and I still get the 32 code and ses light within a few minutes of driving. A buddy of mine who is an old time hot rodder suggested the mild cam may be the issue. Hence, the post. I do appreciate all of the responses.

Peace!!
B2

I doubt the camshaft is the problem. I'd be tempted to check manifold vacuum with a gauge, just to see where it's at.

quote Originally posted by BAM-BAM: A buddy of mine who is an old time hot rodder suggested the mild cam may be the issue.

Perhaps you misunderstood him. As indicated several times in this thread, more likely it's a wild cam that would cause low manifold vacuum at idle.

Perhaps I did, that is entirely possible. I am getting ready to do a 7730, DIS and digital ERG. While I am at it, I plan on changing the cam out to a Crower 03043. I will be able to have the MEMCAL programed to compensate. In the process, I will be regasketing the entire top end. I don't have any high idle issues just a rough initial start. Other than that, this car runs and idles very smooth.

Peace!!
B2

quote Originally posted by BAM-BAM: ... I still get the 32 code and ses light within a few minutes of driving.

That narrows the problem considerably, to just the EGR solenoid itself, the vacuum switch piggybacked onto the solenoid, or the vacuum lines associated with the solenoid. A manifold pressure (i.e. vacuum) gauge will confirm that you have adequate vacuum. The purpose of the vacuum switch is to signal the ECM that it is "seeing" modulated vacuum from the solenoid whenever the ECM is commanding EGR. If the switch isn't working correctly the ECM "thinks" that the solenoid isn't working correctly and sets a DTC 32.

I smell what you're cookin Mr. McInnis. I have replaced the solenoid three different times (quite expensively) and I have had the same result. As I mentioned previously, I am doing some upgrades with the 7730 which should elimenate the issue entirely. The only other possibility is the ECM itself; however, that too will be rectified with the 7730.

Peace!!
B2

[This message has been edited by BAM-BAM (edited 05-27-2013).]

As a diagnostic test you can jumper the two pins of the EGR vacuum switch. If the DTC 32 goes away, then you know that the ECM and vacuum switch wiring are OK. You say that you've replaced the EGR solenoid several times. Have you replaced the vacuum switch too? Check the short vacuum hose between the solenoid and the switch.

Consider also that EGR is active only while cruising at partial throttle, not at idle or at large throttle openings. This alone should make the cam a non-issue.

When I replaced the solenoids, the vac switch was part of the unit. I believe that I have done the jumper, but I am willing to go it once again.

Peace!!
B2