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Old 04-11-2009, 01:36 PM
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EGR question.

I`ve posted before about the mods did to my 92 4.3 V6. When I first put it back together and had the EGR connected it would hardley run, I thought the valve was bad so I left it disconnected. Since the price of fuel went high and I could only run 93 octane, I decided to reconnect the EGR after testing it with a vacuum pump, it tested good. After reconnecting it I noticed it had a smoother power band, and I could run 87 octane without rattling problems.
The only issue is, when I installed the valve I installed the same size orifice as the original valve, but since I`ve did so many mods to the engine I`m thinking it`s too big. The problem is it surges, right dead off idle, cruising in 4th or 5th gear (5th being worse) you can have the pedal pressed less than a 1/4 inch off idle and feel it surging. I`m thinking it`s getting too much exhaust gas and causing unstable combustion. The truck has a bigger isky computer compatible cam, 1.6 ratio roller rockers, 2 pipes running into a single turbo muffler with 2 pipes out, around 9.8:1 compression, Venom TBI injectors, K&N filter, double roller timing set, swirl vane heads with ferrea necked down valves. Edelbrock performer TBI intake, fuel pressure set to 13 psi, MSD wires and coil. Since the mods I`ve made on the exhaust side plus the cam I`m thinking it`s getting too much exhaust gas and this is causing it to surge, since I don`t know much regarding these systems, I could use some help figuring it out. Any help is greating appreciated.
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Old 04-11-2009, 02:55 PM
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Do you have anyway of checking the exhaust gas readings? It could be running either rich or too lean condition, which may be causing your surging. The other thing I would look at is the timing. I don't think the EGR valve will cause that since you did a vacuum check and it appears to be OK. Sometimes they get carbon built up inside, but don't think that would be the problem either. Perhaps retarding the timing may help. It sounds like you have a good setup and I'm sure it's something simple. We just went through this with my next door neighbors 5.0. We thought is was too rich of exhaust gas as well, but after putting a guage to exhaust, it was in the normal range. It ended up being timing issues on his. Good luck.
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Old 04-11-2009, 06:08 PM
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Quote:
Originally Posted by DoubleVision
I`ve posted before about the mods did to my 92 4.3 V6. When I first put it back together and had the EGR connected it would hardley run, I thought the valve was bad so I left it disconnected. Since the price of fuel went high and I could only run 93 octane, I decided to reconnect the EGR after testing it with a vacuum pump, it tested good. After reconnecting it I noticed it had a smoother power band, and I could run 87 octane without rattling problems.
The only issue is, when I installed the valve I installed the same size orifice as the original valve, but since I`ve did so many mods to the engine I`m thinking it`s too big. The problem is it surges, right dead off idle, cruising in 4th or 5th gear (5th being worse) you can have the pedal pressed less than a 1/4 inch off idle and feel it surging. I`m thinking it`s getting too much exhaust gas and causing unstable combustion. The truck has a bigger isky computer compatible cam, 1.6 ratio roller rockers, 2 pipes running into a single turbo muffler with 2 pipes out, around 9.8:1 compression, Venom TBI injectors, K&N filter, double roller timing set, swirl vane heads with ferrea necked down valves. Edelbrock performer TBI intake, fuel pressure set to 13 psi, MSD wires and coil. Since the mods I`ve made on the exhaust side plus the cam I`m thinking it`s getting too much exhaust gas and this is causing it to surge, since I don`t know much regarding these systems, I could use some help figuring it out. Any help is greating appreciated.
Did you put a new program in the computer chip? Not to say the EGR doesn't need trimming. But the computer program cannot deal with more than about a 10% change in mass flow which with TBI/MAP systems it's computing from sensor information. The so called learning the computer is capable of is really nothing more than mixture adjustment based upon O2 sensor input. At best this is 10% and that assumes the engine ran on the bottom side of the nominal range to start with.

The changes you've made can easily exceed the computer's ability to adjust. These would be mostly in the cam specs, 1.6 rockers and exhaust system. These type changes modify the sensor relationships of Manifold Absolute Pressure (MAP), to throttle position, to RPMs. This is how the computer calculates Mass Air Flow (MAF). When these relationships it's expecting to see are different, it incorrectly computes MAF and looks to a schedule in the fuel and ignition advance map that doesn't correlate to the engine's operating condition. These OBDI systems are less of a computer and more of a fancy calculator, so they aren't to hard to foul up and outside the closed loop O2 sensing system have no means of correcting errors without reprogramming the sensor data to new correlations with the function maps.

Bogie
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Old 04-11-2009, 06:17 PM
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No Bogie, I never did get a chip for it. Since The economy has put me out of work I don`t know when I`ll be able to afford it. I was thinking on removing the 1.6 rockers and installing the stock rockers which ratio is really 1.4. I thought that maybe it would help vacuum somewhat. I know higher rocker ratios really don`t do much but add lift, I also know they increase duration by maybe 2 degrees. Should I detune my engine to get it back within it`s learning limits?
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Old 04-11-2009, 06:28 PM
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The EGR valve only opens when it is told to. If you have a new EGR valve, the culprit is probably elsewhere. A slight opening of the throttle is not the place the EGR opens anyway. You need to look elsewhere for the answer for your driveability problems, I'm thinking too much cam.

The so called computer compatible cams aren't always. With OBDI and MAP, you are stuck with a narrow set of fuel tables that are not tolerant to cam changes. The OBDII/MAF system is more cam change tolerant.
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Old 04-11-2009, 08:39 PM
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Make sure you have the valve connected to the correct vacuum source per the emisions diagram.If it is on the wrong port or vacuum switch it can open at the wrong time causing this.Also if it is a replacement valve they do come with various size restrictors to taylor them to different applications.This is so they can make one high flow valve for larger engines and restrict it down to use on smaller ones instead of building different valves for every application.
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Old 04-12-2009, 08:17 AM
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You could drill and tap the EGR valve for a 1/2" pipe plug and insert a drilled brass pipe plug restrictor for testing, might be worth a try. Use the plugs that have the recessed Allen head.

I had a similar problem with a 400M and a mild cam, I ended up with a 1/4" hole in the restrictor to avoid the surging.
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Old 04-13-2009, 09:22 AM
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Quote:
Originally Posted by DoubleVision
No Bogie, I never did get a chip for it. Since The economy has put me out of work I don`t know when I`ll be able to afford it. I was thinking on removing the 1.6 rockers and installing the stock rockers which ratio is really 1.4. I thought that maybe it would help vacuum somewhat. I know higher rocker ratios really don`t do much but add lift, I also know they increase duration by maybe 2 degrees. Should I detune my engine to get it back within it`s learning limits?
Changing the rockers could bring you back, 1.6 against a 1.4 ratio makes the cam look 14% bigger to the engine, that's quite a bit.

Bogie
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Old 04-13-2009, 09:39 AM
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Thanks Bogie, as soon as the weather clears up again I`ll tackle it.
Also thanks to Chuckie, I`ll play around with orifice sizes until I find one that doesn`t create a surging issue.
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Old 04-13-2009, 10:14 AM
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You are going about this in the worng way. First, you need to determine when the EGR opens and how it relates to your drivability issues.

I think you were saying that when driving with the EGR disconnected, the engine runs fine. Does it?

Put a tee in the vac line going to the EGR valve and hook a vac gauge to it. Do you have vacuum when the engine is revved cold in park? (1-2" is OK and may appear to pulse)

When the engine is warm and on the road (closed loop), when does the vac gauge indicate the EGR is getting a vacuum signal? You may want to also have a OBD1 code scanner/reader hooked up during the road test to see what the computer is doing. It may not be in closed loop at all.

Then, one way or another you will know if it's EGR related or not. To correctly determine which oriface to use, you need an exhaust gas analyzer. The whole purpose of the EGR system is to lower NOX by reducing combustion temperature. The engine is designed to run properly with the correct dilution of exhaust gas. The analyzer will show you whether or not the orifice is right.

I still think your cam is too big for this engine. Remember, you have OBD1, not OBD2.
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