Stuttering problems!!

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.

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.

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.

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.

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?

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.

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.

Mikado, Do U Know What The Egr System Does?

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.

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.

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.

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.

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?

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.

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.

That's fine , you are lucky.

Most people that put a restrictor plate will experience the check engine light coming on until they drill the holes out enough.

Just for an experiment, remove the plate, reset the computer and try it without it.

the computer doesnt throw a code because it doesnt think anything is wrong with the egr system. when the computer activates the egr valve fuel and timing are changed to compensate for the egr gasses going into the engine. you blocked off the egr. the computer doesnt know it. it thinks egr gasses are flowing through. it changes fuel and timing not knowing there is no egr gasses in the engine. hence your stumble/stutter. you have to remember the ecm is dumb is is reading off of a program. there is nothing in there telling it that you blocked of the egr.

Then if what you are saying is true, the O2 sensor will not change fuel other than by what the LEGR is doing, the knock sensor will only change timing based upon the LEGR only, the MAP sensor only measures the barometric pressure to check the operation of the LEGR, etc, etc.

If what you are saying is true, then how did the poster drive 20,000 miles without a problem? I haven't seen anyone explain that yet. Is the computer so slow that it took that long?

Back before there were any O2 sensors, MAP, TPS, fuel injection as stock and all the other sensors, you had PCV's, air pumps to inject cold air to burn off unspent HC's, heat riser valves and vacuum operated EGR's. This would have been the late 60's to early 70's. And when those EGR's would plug up with carbon, and they did quite frequently, standard operating procedure was to clean them, and plug off the vacuum line. They still ran and performed fine.

The LEGR is not unique in sending a signal to the ECM. Ford has been doing it for years with their EEC-IV with a plunger mounted on top of the EGR. They can be plugged off as well and the engine will operate fine. The computer will recieve the signal, the ECM believes it is open, the O2 monitors the exhaust gases and will adjust the fuel delivery based upon the position of the throttle, the vacuum in the manifold, the incoming air temperature etc. The purpose of the EGR system is to reduce the NOx only which is the main culprit in smog.

We see the EGR at times to be a problem but that is due to it not closing properly and creating a vacuum leak or in the case of vacuum operated another vacuum leak if the diaphragm is ruptured. In the case of an EGR being stuck closed, your looking at a failing condition in exhaust gases during an emission testing depending upon the acceptable levels being looked for.

Ok. I have checked all my connections, put the EGR back in service by removing the block off plate, pulled the computer and check those connections. I drove it about 200 miles without a problem and then tonight on my way home from work it was doing it again. No codes were stored in the computer. I am wondering if the computer itself is going bad??? The only thing common with the TPS and the MAP is that they are both supplied with the same 5volt reference from the computer, not the same ground like I previously thought. I am running out of ideas!!!

One other thing, does anybody know where the ESC bypass connector is on a 93 CPI. I can't seem to find it and I am about to cut the wire to check my timing.

One other thing, does anybody know where the ESC bypass connector is on a 93 CPI. I can't seem to find it and I am about to cut the wire to check my timing.

Pull your carpet back under the drivers side up under the dash. It is a small (approximate) 5/16" connector with brown/white tracer. Can't miss it.

Do you have a scanner that allows you to read the volts to the tps? If not you can back probe the connector than slowly sweep the tps slowly while looking for a smooth increase in the voltage. It should start below one volt, I believe usually around .60 volts and go up to around 5 volts give or take. It you find a flat spot then the tp is bad and confusing the ecm.

I must really be blind because I have looked all under the driver side dash and still can't find it. Is the Brown/wht wire the only wire in the connector?

I thought it was on the passenger side under the heater box at carpet.

I must really be blind because I have looked all under the driver side dash and still can't find it. Is the Brown/wht wire the only wire in the connector?

I am sooooooooo sorry BluBlazer. I done got my right and left all discombobulated.

It is the passenger side and it is easy to find. I was thinking passsenger and don't have a clue why I typed driver's. Must have been a bad morning.

Ok....I think I found my problem..the two wires going to the CPI unit in the intake from the external engine harness must have a break in them. Everytime I touch the wires with the engine running it stumbles and even kills it. Now, I need to find the harness so I can replace it. I would use regular wire, but these wires are teflon coated. The dealer wants $70 dollars for the harness. OUCH!!!!

Just replace the wire.

You don't have to replace the whole harness.

The harness that I am speaking of starts at the CPI unit with a connector and goes from the CPI unit through the intake plenum and out to the main engine harness to another connector. There are only two wires in this harness and what makes them special is that they are Teflon insulated so they can stand up to the conditions inside the intake. The total length of the two wires is only about one foot. The trouble that I am having is finding teflon insulated wires. Once I find the wire, I am just going to cut the old wires off of each connector and attach the new wires to them. The dealer wants $70 bucks for the new wires and connectors.

Yea, that is what I would do.

Or maybe use the shrink tubing on them.

The harness that I am speaking of starts at the CPI unit with a connector and goes from the CPI unit through the intake plenum and out to the main engine harness to another connector. There are only two wires in this harness and what makes them special is that they are Teflon insulated so they can stand up to the conditions inside the intake. The total length of the two wires is only about one foot. The trouble that I am having is finding teflon insulated wires. Once I find the wire, I am just going to cut the old wires off of each connector and attach the new wires to them. The dealer wants $70 bucks for the new wires and connectors.

How about looking in a bone yard?

Go to www.mcmaster.com
Scroll down to "Lighting and Electrical"
Click on "Wire and Cable"
At the top of the web page you can change catalog pages - Go to page 713.
Here you will find Teflon coated wire (called PTFE coating). Pick what you need and it will be at your door in a couple of days. Just make sure you pick the right temp range and wire gauge.

Thanks for the information, but I just went to CarQuest and got the connector and pigtail for a TBI injector, includes and new connector and the Teflon coated wire all for $12 bucks. The connector is the same for a TBI injector as a CPI unit. A little soldering into the Engine harness and problem solved. I drove it around for about 30 miles and no problem. I will post the long term results in a couple hundred miles. Thanks for all your help!!!