Stuttering problems!!

[BluBlazer]

My 93 Blazer 4.3L CPI started stuttering the other day when I was about an hour and a half into a trip on the highway. I noticed that if I kept the throttle at the same position or closed it, the stuttering would get worse, but if I stepped on it and accelerated it would go away. I have replaced plugs and wires, cap, rotor. I got three codes out of the computer, TPS, MAP, and Lean Condition. I have replaced the fuel pump a couple of months ago. I checked the output of the MAP and TPS and they show alright. My EGR is blocked off and has been for the last 20,000 miles. The next time I drove it, it ran just fine, but then last night it started doing it again. I do almost all highway driving and have not had a problem up to this point. I checked fuel pressure also and it was right around 60 psi. I can't seem to figure this out...and I don't want to just throw parts at it in hopes of fixing it. I also checked the timing and it is right on 0 degrees like it calls for. Anyone have any ideas...I also changed the fuel filter when I changed the pump.

[busa_4]

your egr system is programmed to only open when you are cruising. it will not open at idle or W.O.T.. since you dissabled it, the computer doesnt know, so it is making fuel and timing calculations based on the egr gasses being sent into the intake. your egr gasses are not flowing so your fuel and timing curves are not correct causing the stumble.

[BluBlazer]

The EGR Valve is still hooked up and bolted inplace, the only difference is that there is a plate between it and the intake eliminating any Gas flow. I have had it this way for over a year now and this is the first time I have had any problems.

[BlazerLT]

Remove the plate.

The EGR system is part of your engine's system and is required for proper running.

Your computer needs it.

Also, check for a leaking CPI injector.

[Mikado14]

Originally Posted by BluBlazer

The EGR Valve is still hooked up and bolted inplace, the only difference is that there is a plate between it and the intake eliminating any Gas flow. I have had it this way for over a year now and this is the first time I have had any problems.

First off, the engine should run fine with the EGR plugged off. How many times has someone unhooked the vacuum line and plugged it and the EGR does nothing? However, you have an LEGR on your engine and I am assuming that you still have the wires plugged in. The only problem of major concern is that your vehicle will not pass an emission inspection, if that is not a worry, then don't.

A few questions,

Does the vehicle accelerate as it always did?

Does it idle smooth?

Are cold and hot startups OK?

How did you check the outputs of the TPS and the MAP?

When was the last time you changed your fuel filter?

Why did you plug off the LEGR in the first place?

[BluBlazer]

Thanks for the reply, I agree with you that the EGR has no performance implications other than it increases exhaust emission. I am fortunate enough to live in a area that does not do emission testing. I still have the EGR plugged into the harness and it is still functional except that there is no Exhaust flow through it because I got tired of cleaning the carbon out of it everyweek and causing the engine to not idle. The EGR has been blocked off for about a year now and I have had no other problems or a change in preformance. The vehicle idles just fine and accelerates just as it always has. Cold or Hot startups are not a problem, the engine fires right up immediately. I checked the TPS and MAP sensor by reading the signal voltage with a volt meter and for the TPS sweeping it from closed throttle to WOT, for the MAP, I measured voltage while changing the amount of vaccum applied to it through a hose and a hand vaccum pump. I changed the Fuel filter about two months ago when I changed the fuel pump. I have no problem accelerating, in fact the stuttering clears up when I accelerate from a constant highway speed.

[BlazerLT]

The digital EGR can not be compared to the vacuum based models.

The digital EGR requires a feedback signal that the computer monitors. Without it, it will trip an engine code.

[busa_4]

Mikado, Do U Know What The Egr System Does?

[BlazerLT]

Here is a good explanation:

http://www.autoshop101.com/forms/h61.pdf

What could be happening is detonation from increased combustion chamber temperatures creating detonation which will cause the knock sensors to retard the timing.

[Mikado14]

Originally Posted by busa_4

Mikado, Do U Know What The Egr System Does?

Yes I do. It's purpose is to reduce the oxides of nitrogen that are produced from high combustion temperatures. The idea of recirculating the inert gas from the exhaust, mainly the carbon dioxide and other crap, is to cool the combustion chamber and not produce the NOx. If the EGR remains closed, as since he has it blocked off, it will cause the combustion temperatures to rise and will cause pinging or knocking. A way around that would be to run higher octane fuel, however, the knock sensor should be adequate enough to retard the timing to eliminate the knock and the engine should perform acceptably.

In the event the EGR is plugged open it merely acts like one big vacuum leak.

Here in Pennsylvania for emissions, three components of the exhaust are checked on pre-OBDII vehicles that have OBDI. They are CO, HC and NOx. If the EGR is closed off, the NOx will go beyond the pre-established limits as set forth by the state.

[Mikado14]

Originally Posted by BlazerLT

Here is a good explanation:

http://www.autoshop101.com/forms/h61.pdf

What could be happening is detonation from increased combustion chamber temperatures creating detonation which will cause the knock sensors to retard the timing.

LT, I agree, the knock sensors should do their job.

[BlazerLT]

True, but they are not meant to compensate for extreme circumstances where the EGR gas flow is completely blocked off.

This could be causing the studdering, the knock sensors can not compensate enough.

[BluBlazer]

The question I have is why would I get a TPS, MAP, and Lean Condition code from blocking off the EGR almost a year after it was blocked off? I have put almost 20,000 miles on the truck since I blocked it off and this is the first time that I have had any issues. One thing that i have found out is that the TPS and MAP sensors both share the same ground and I know the truck will not run without a map sensor input to the computer. I am starting to think that it might be a loose ground which would also explain why it is a somewhat random problem. What do you guys think?

[Mikado14]

Originally Posted by BluBlazer

The question I have is why would I get a TPS, MAP, and Lean Condition code from blocking off the EGR almost a year after it was blocked off? I have put almost 20,000 miles on the truck since I blocked it off and this is the first time that I have had any issues. One thing that i have found out is that the TPS and MAP sensors both share the same ground and I know the truck will not run without a map sensor input to the computer. I am starting to think that it might be a loose ground which would also explain why it is a somewhat random problem. What do you guys think?

You didn't get the code from the blocked EGR. You theory is good. Start checking and cleaning all the grounds you can find.

[BluBlazer]

I didn't get a code for the blocked EGR because the EGR is still plugged into the harness and is functional as far as the computer is concerned. The computer does not have anyway of knowing EGR flow, just the position of the Valve. The Valve still opens when the computer signals for it to open, but there is no flow through it because there is a plate between it and the intake.