My '94 w/ 302 started running crappy about a month or so ago, not a sudden change in performance just a gradual decline. Also the check engine light would come on after driving about 75 miles at 70 MPH. Changed spark plug wires, cap and rotor (all were old), also found the timimg to be retarded by a few degrees. Setting the timing made a big difference but the CEL was still coming on after a long drive. Pulled codes... 334 KOEO and 334 and 157 in memory. Replaced MAF and found the EGR valve to be full of carbon, the diaphram leaking and the pintle not seating properly, the valve was also stuck closed. Replaced EGR and EVP, this made the engine run worse! When accelerating the rpm climbs to about 1500 and it "stumbles" unless the accelerator is pressed hard. I have figured out this is being caused by the EGR opening. It's really bad when the a/c is on. Blocked off the EGR valve.... WOW the truck runs better than it has in the 3 years I've owned it... quick throttle response and POWER!

I connected a vacuum guage to the port on the EGR valve... there's no vacuum present at idle but slight vacuum when the engine is revved. When accelerating there's 1-2" of vac at 1100 rpm and 3-4" at 1500 rpm unless the throttle is opened further at which point the vacuum drops to zero until the throttle is released some.

When should the EGR open? I read it doesn't open until cruising speed is reached. Is it supposed to open during "light" acceleration from a stop? Any ideas what to do next? She's running great with the EGR blocked off but I would like to return it to it's original configuration.

 

I replaced my EGR valve and I still get the CEL. I've been told it could be the tubes connecting to the EGR valve from the Manifold. I haven't removed them to clean or blow out with the compressor but you just reminded me to do so...

I am assuming it should open when you accelerate to do its job "Recycle Gas".

-Stranger

 

Code 157? No such animal that I can find anywhere. Can someone shed some light on this one for me?

If the EVP has been replaced and Code 334 is still tiggering, check the wiring to the EVP for damage. The computer "believes" it is getting a higher voltage from the EVP than it should which is why the vacuum supplied to it is not opening it at the proper time. The EGR is only supposed to open during "closed loop" operation which, roughly translated, is whenever the engine is warm enough and not being taxed enough to worry about additional exhaust gasses being dumped back through the combustion cycle. Try running the KOEO test with the EVP sensor disconnected and everything else operational (unblock the EGR valve so it can work as it should). If Code 334 is still present, check and repair the wiring from the EVP sensor to the ECA (computer). If Code 328 is received instead of 334, the EVP sensor is still the culprit. If 334 Returns AFTER replacing the sensor, and there is no wiring damage, there is a software issue within the ECA. Throughout this process make certain you clear the codes after each repair/replacement or you will run the risk of memory codes still coming up even after the problem has been rectified.

 

157 was in memory... "MAF sensor went below 0.4 volts during the last 80 warm-up cycles". I figured it was time to replace the MAF anyway as it had a big crack in the plastic cover on top.

334 has not returned since I replaced the EGR valve and EVP. The valve was pretty much shot and I figured for $20 it'd be a good idea to replace the EVP as well. Cleared all the codes and none have returned (KOEO/KOER or memory) so I figured it should all be working and running well, this is not the case however. It was showing 111 so I assumed it was going to be happy.

I read that the EGR valve opens fully at 4" of vacuum and I'm seeing 3-4" on the port of the valve during acceleration when the RPM reaches 1500 so the valve is opening and killing acceleration unless I mash the pedal down, then the vac drops to zero and it takes off. It seems that the valve is opening either too much or when it shouldn't be. Should it open at all when accelerating? There's no vacuum at the port at idle so I'm assuming the solenoids that control the EGR are not leaking by.

 

There is a soleoid the controls the flow of vacuum to the EGR valve. If it has failed and is stuck open then the vacuum flow to the EGR fluctuates every time rpms change. The reason for it closing under hard acceleration is the drop in manifold pressure. There is no EEC-IV minotoring of the EVR solenoid so it won't show up as a fault code of its own.

 

Thanks for the info. I was leaning in that direction but as it's not getting vac at idle I was thinking it's probably not stuck wide open but vac could be leaking past it. I'll check it out, the old girl's got 280,000 miles on her so it's probably due for a new one anyway. I'll tell ya with that EGR valve blocked she runs better than ever, it's always run good but always has had a slight "stumble" around 1500 rpm when accelerating, I never thought anything of it though. I will check out the solenoid this weekend after the 100+ degree heat wave breaks and let you know what I find. We're having what been referred to as a heat storm here in NJ.

 

Its been 98+ here in St. Louis since the storms that made the national news a couple weeks ago... I feel your pain.

 

Well I replaced the egr control solenoid still it does the same thing!!! Argh! Drove out to Pennsylvania (100 miles) about halfway there the CEL came on, went off, came on, went off, came on... when I arrived at my destination I pulled the codes. Got code 172 (running lean, right side). Checked fuel pressure, it's showing 35 PSI. Changed the fuel filter (was somewhat restrictive) also replaced pressure regulator (wasn't holding vacuum). While driving back to Jersey CEL came on again... I'm reaching the end of my rope with this thing! I just replaced the O2 sensor... we'll see what happens. Around town it's not generating codes, just on long trips. Also I've noticed a "whistling" sound (exhaust leak???) from under the truck when accelerating which comes and goes (mostly before everything warms up). It's gonna drive me nuts!

 

I have a sure fix for the CEL. Black electrical tape. It works great on the highbeam indicator also.

 

Believe me I've thought of that but I want the thing to run right and I have to go though inspection soon and pass the emission test.

 

Make sure the new O2 sensor is fully seated. Your whistle could be there. While you are checking things make sure the check valves on the Thermactor system are not cracked of broken in half. There are two. One is mounted to the upper end of the metal tube extending from the catalytic converter and has formed rubber hose running up to the diverter valve on the pass. side of the engine way in the back near the firewall. The other is mounted in the metal crossover tube that runs across the back of the engine between the cylinder heads. A defective check valve will give you exhaust leaks, strange noises AND if the one in the crossover tube is broken there will be more O2 than needed getting into the heads than the computer is expecting at SOME times... hence the ON, OFF, ON, OFF with the CEL. Since the computer controls the flow of fresh air from the Thermactor pump (smog pump) into the heads, a leak between the diverter valve and the check valve would yield this type of problem.

 

I made sure it was nice -n- tight when I installed it. The old one wasn't tight at all. I 'll check out those hoses and check valves... hopefully I can get back there without killing myself... lol. Evrything else on the motor is pretty easy to get at but not them things.

I have a question about the O2 sensor... in the Hayes manual it says the voltage out of the sensor should be steady between .35 - .55 volts. The old O2 sensor was flucuating almost a full volt. The one I installed also flucuates but not as bad... it's between .30 and .80. I read somewhere on the net the voltage should flucuate. Which is right? The wiring and the connectors look ok and it's getting battery voltage with the key on like it should.

 

The current genereated by (yes generated by) the O2 Sensor is somewhere between 100 and 900 MILLIVOLTS. The crossover voltage is the midpoint (450 mV). Any reading above this number is interpreted by the computer as running lean and any reading under 450 mV is considered rich. The reading needs to remain fairly constant for this reason. The O2 sensor will NOT generate voltage until it reaches about 600 degrees F so don't bother testing it unless the engine is at operating temperature. HEGO sensors are slightly different in that they have an internal heating element to maintain temperature regardless of exhaust flow. This however does NOT change the fact that the reading should remain fairly constant. The whole reason the O2 sensor produces voltage at all is because the chemical reaction within the sensor at temperature causes the sensor itself to become conductive to oxygen. Send more oxygen to it and it produces more voltage.

 

That's interesting... the old O2 sensor was giving widely varying voltages, it was just all over the place. With the new one the variation is not as much but still is not steady or anywhere close to it. It constantly varies between about .30 and .80 volts. It goes high then low, high then low, high then low which I would take to mean the computer is seeing as lean then rich, lean then rich with no happy medium. Seems like it's going from one extreme to the other and back all within a matter of seconds, very strange.

I inspected all the Thermactor components behind the engine as best I could, I couldn't see or feel anything back there that appears to be broken or cracked. Pulled a vac on both the vac actuated valves and both hold vacuum. Pulled vac on the two solenoids that control the valves, neither of those seems to be leaking either.

 

This thing's beatin' me down... lol. I do want to thank you for your help though!