I have a 1994 STS with 134,000 miles on it. I have been having problems with my car running sluggish and surging after it warms up and the computer goes from "open loop" to "closed loop" and starts reading from sensors. I have no codes, current or history, and am currently at a loss. I have suspected the EGR, but have cleaned it on the suggeston of some here and also used the diag in the manual to test via onboard computer and it passed on 3 seperate occasions. During the test there was a considerable change in engine behavior so I must conclude that the EGR valve is operating properly. I looked at the pressure regulator and I don't see any fuel from the vacuum side but don't know how to test further. The car seems to run good once I get over 1700 rpm, but as it runs at highway speed and I am not in the power the car will shake/surge quite a lot. Undeer acceleration there seems to be good power, as I get a 2nd gear chirp from the tires when I get on it. I have had to replace the rear O2 sensor about 6 months ago, but that was giving me a code and all seemed well after that replacement was made. I have had this car for 8 years and it has been a gem until now. I don't know where to go at this point. Any help or suggestions would be greatly appreciated.

 

Clean the throttle intake and report back ...


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Stomper, thanks for the reply. I cleaned the tb and decided it would be a good time to replace the air filter too. I can't say it made a significant difference for the better, but maybe a little. Under heavy acceleration it seems normal. The problem exsists under normal acceleration, where I can feel a lack of power, but it is most present when the car is at highway speeds (60-65) and I let off the accelerator and the engine coasts. This is when the shaking and jerking is at its worst. Thanks for the continued help.

 

very good two things to do.

now, have you checked
fuel pressure? new fuel filter? plug wires? fouled plugs?






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Thanks again for the suggestions. I have already replaced the fuel filter. I tried to check pressure yesterday, but I couldn't get my gauge on the rail port. The cruise control diaphram is in the way. I will check that too. I wanted to ask about the EGR system. I was driving the car yesterday and while at highway speed, I can override the egr pintle position and keep it full closed and the engine seems to run good, no jumps or shakes. If i manually open to 5% or so I get the same jumps and shakes that are present when I let the computer handle things. I have also noticed by just monitoring the pintle position that it jumps from open to closed rapidly when the shaking begins to occur. This is leading me to think that the egr valve is somehow involved in the problem. I have already run the test on the valve and it passed with similar numbers on 3 seperate occasions, but it could still be bad, or maybe the valve is doing what it is told to do, but it is getting bad info. I haven't yet determied what sensors the pcm uses to tell the egr what to do, but any advice along this front will be appreciated as well. I will check the fuel pressure and plugs/wires as suggested and post back progress.

 

Follow up: I was able to test the fuel pressure, range from 38-42 psi, seems normal. I have recently replaced the spark plugs and wires, but I did pull 2 plugs on the front and 1 on the back checked the gap, very near .05 (what is on my engine sticker) and compression, 120 psi on all three. Next I did the power balance check (turn off each injector one at a time and check rpm for drop and engine for rough idle) start for each 1250 drop to 1225 and rough idle on the drop of each cylinder. Each was nearly the same and there was a notice of rough idle. I then looked at the idle speed controller and ran the test on it. It was borderline out of range (tp change by 9.8 degrees between 10 & 11 was range) modified this to get 10.5 degrees change and then ran tp sensor/idle learn procedure. I did notice a possible problem when the iac controller would fully extend there was a pop sound followed by a retraction in the controller, but then it would procede to full extension. All done with no change in problem of surging/jumping. Still seems to be under light acceleration, but goes away with heave acceleration. Thanks in advance for any suggestions or help

 

you have done a great job doing diagnostics.

excessive EGR input can cause misfires and partial burns .. and .. shakes?

EGR is typically not employed at high loads (acceleration) because it would reduce peak power output. This is because it reduces the intake charge density.

EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle.


I dont have a manual on your car but this is a write up from my archives, source unk.

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the single diaphragm EGR valve. It consists of a spring-loaded diaphragm that is connected to a pintle and seat by a slender steel shaft. Normally closed by spring tension, as it receives ported vacuum, the diaphragm rises, which pulls the pintle off its seat and enables exhaust to flow into the valve's chamber and then on to the intake manifold. To test this component, use a hand-held vacuum pump connected to the vacuum nipple to raise and hold the diaphragm. About eight inches of vacuum should do the trick. The valve should hold vacuum and raise the pintle in a linear fashion. When the engine is idling, pumping it up should stall the engine. This type of valve may or may not have vacuum modulation. Remember, vacuum modulation to the EGR is a vital ingredient of good driveability and precise NOx control. This type of EGR valve is used with a thermal vacuum switch and maybe an inline vacuum delay valve.

The positive back pressure EGR valve can be identified by the letter "P" stamped next to the part number and date code. A back pressure valve is easy to spot because its pintle shaft is much thicker than the single diaphragm type. This is so because the shaft is hollow. The hollow design allows exhaust gases to flow into the shaft and push up on it. When positive back pressure in the exhaust system is sufficient, the shaft raises up and seals the built-in control valve. Once the control valve is closed, it allows applied vacuum to pull up on the diaphragm. Without back pressure to lift the hollow shaft and close the control valve opening, the EGR valve will not hold vacuum. It is bled off to the atmosphere. This design thus modulates EGR flow by modulating the applied vacuum. As engine load increases, so does engine back pressure, which causes the control valve inside the EGR to trap vacuum and open up. To test this valve, bring the engine up to 2,000 rpms to create back pressure, then apply vacuum. EGR should open and cause a 100 rpm drop or more. Exhaust leaks or a modified exhaust system can create havoc here. Adding dual exhaust or headers on a car designed for a single exhaust will reduce back pressure and set a Code 32 on GM cars. Positive back pressure EGR valves are used in simple vacuum controlled systems, as well as more complex pulse width modulated applications.


EGR solenoids are used with all types of EGR valves, especially back pressure type valves. The EGR solenoid will have two or more vacuum lines and an electrical connector. The solenoid also has an air bleed and sometimes an air filter. Vacuum is bled off through the filter vent. The PCM uses the solenoid to regulate vacuum to the EGR valve. The vacuum can be manifold or ported vacuum. The solenoid is a vacuum switch with inlet and outlet vacuum ports. The PCM calculates intended EGR flow from various other inputs and then sends a pulsed "on/off" signal to the solenoid. No vacuum flows until commanded by the PCM. This signal turns the vacuum on and off in rapid succession. This is called "pulse width modulation." If the filter becomes clogged, the vacuum cannot bleed off and too strong a signal will be sent to the valve. If that happens, the EGR valve will open too much and cause a driveability problem.

Remote vacuum transducers
All manufacturers use them, some more than others. Shaped like a flying saucer with three or more vacuum ports, they modulate vacuum by using manifold and ported vacuum against each other along with an exhaust back pressure input. The result is a carefully controlled vacuum signal to the EGR valve that is mechanically modulated by engine load. Your best bet with these is to study the vacuum diagram on the underhood emissions label. Make sure the vacuum hoses are in good condition and properly routed. Many of these units have air filters also. You can clean them out to prevent too much EGR flow.


The negative back pressure EGR valve is identified by the letter "N" and looks similar to the positive back pressure EGR valve. The valve is opened by a combination of applied engine vacuum to the control valve and negative exhaust system pulses that happen as each exhaust valve closes. As soon as the pintle opens, back pressure is reduced slightly, which opens a control valve vacuum bleed and then the valve quickly closes. In this manner, EGR is modulated by negative exhaust system pulses. To test it, apply vacuum with a hand pump when the engine is off. The valve should open and hold vacuum.



electronic/mechanical EGR valves
This type of valve has different names with each manufacturer. It is easily identified because it has a single vacuum source inlet and a three-wire electrical connector. Mechanically, it operates like a single diaphragm EGR valve with a twist. It has a pintle position sensor riding atop the EGR diaphragm. This tells the PCM the amount of EGR valve opening as it is actuated. The PCM then commands a pulse width modulated solenoid to apply an appropriate amount of vacuum on-time. GM makes one of these units that has the integral pintle sensor and an integral solenoid with air filter. The only separately serviceable part is the air filter. Ford, Honda and Mazda all use a variation of this design with remotely mounted solenoids. The idea behind the pintle sensor is to give the PCM precise feedback as to exactly where the EGR valve is positioned. The PCM can then modulate the vacuum signal to it accordingly. The pintle position sensor is a potentiometer. Like a throttle position sensor, it is a variable resistor. The wiper arm within the sensor can wear and develop opens in the sensor return signal. A sweep test with a digital volt/ohm meter (DVOM) or scope can be used to test the sensor. The PCM has an internal "map" of where the pintle sensor should be at any given time. If the sensor's voltage reading is too high or low, a trouble code will be set.

GM's Code 32 has been around a long time and can be caused by a variety of reasons. Every three to five years, the PCM strategy on this code changes. Make sure you review the proper flow chart any time you work on one on these. The strategy is slightly different depending on which engine, transmission, body type or year the car is. The most common strategy entails the PCM looking for fuel integrator counts to decrease momentarily when the EGR is commanded open. Why? There is no oxygen in the inert EGR gas, so the integrator subtracts fuel to compensate. For this to happen you must have a good working oxygen (O2) sensor. O2 sensor checks are usually not in the Code 32 flow chart, so be aware. Newer models and other makes look for a change in manifold absolute pressure (MAP) when EGR is flowing, which is a more reliable method.

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