350350

97 LT1 ~145,000 miles.

Setting codes 171, 174, and 400.

171- Lean bank 1
174- Lean bank 2
400- EGR valve.

I replaced the EGR valve a couple of years ago, and today I tested it again. I put vacuum on it and it opens and holds fine.

I had the engine apart about 12 months ago and the EGR passages aren't blocked. Like new inside.

So the lean banks tell me the valve really isn't opening. But how do I test the solenoid? Could it be anything other than the solenoid? I took both vacuum lines off and inspected them and there aren't any breaks or obvious leaks.

Help!

Injuneer

The lean codes tell you the PCM is adding more than 20% extra fuel using the long term fuel corrections. I'm not sure how you correlate that to a problem with the EGR system.

With regard to the EGR flow detection code, have there been any changes made to the exhaust system recently that reduced backpressure?

I would test the complete EGR system by letting the engine idle, pulling a vacuum on the vacuum hose that connects to the intake manifold, and applying a ground to the EGR vacuum solenoid (gray wire) to open it. The brown wire supplies a constant 12V when the key is in the "on" position. If the engine stumbles when the solenoid opens, you have verified vacuum is reaching the EGR valve, and the EGR system is flowing exhaust into the cylinders. That eliminates problems with the solenoid, the vacuum lines, the EGR valve and the passages.

350350

Thanks Injuneer, that's exactly what I need to know, how to test the solenoid.

For some reason I thought the EGR 'richened' the fuel mixture at some point. But they must be unrelated.

So the entire EGR system seems fine- no broken hoses, solenoid activates properly, and pretty much the exact procedure you outlined makes the engine stumble like you said. As far as I can tell, that entire system is working perfectly.

No recent exhaust changes, as far as I can tell no hint of any leaks. It's got a Loudmouth catback on it but it has had that for years and years. The manifolds, y-pipe, and cats are all stock.

Is there any chance a bad tank of gas would have caused the lean burn codes? And/or the EGR code? I was thinking about it and I got a tank of gas out of town (so just different than my usual gas station) right before it set all those codes. Shooting in the dark on that but it was right at the same time.

350350

A quick update- I cleared the codes and the 400 seems to be gone. Only maybe 200 miles though since I cleared.

The 171/174 are intermittent. Light comes and goes.

Injuneer

Is it possible the corrugated riser tube for the EGR system is cracked, and when the valve opens, it's admitting air to the intake manifold? If it was cracked, it would be pretty loud though.

It takes very specific conditions for the PCM to run the P0400 diagnostic. Has to be a very steady part throttle cruise condition, and the throttle position can not change for several seconds. It's possible you haven't hit those operating conditions.

350350

The 171 and 174 codes have still been intermittent for a while now. This weekend I'm going to take apart everything that I can even guess could cause a vacuum leak, and inspect all the exhaust. I'll check the EGR tube like you suggested.

Is there a way to test the MAF? I have OBDWIZ so I can/could monitor the MAF readings. It was very dirty- note that this car is a 97 with almost 150,000 and it has had daily driver duty more than half of its life. Ram Air hood sucking up rain water and occasional drive-thru car washes too. I cleaned the MAF with the real and official MAF cleaner but it didn't seem to help.

Injuneer

The MAF sensor produces a variable frequency signal. You can read the frequency with many low-$ multi-meters, if they have a frequency scale. I can give you the stock tables so you would know what frequency to look for at idle, WOT at 5,800 RPM, etc. If you can read the MAF flow in grams per second, look for readings around 6-10 gps at idle and 240 gps at WOT/max RPM (unmodified engine).

OBD-II includes two rationality checks that set P0101, where it compares the AF sensor reading to a calculated mass-air flow using the speed-density method. But it only sets a code if the MAF and speed-density readings differ by more than 25% at high flow conditions and 40% at low flow conditions.

350350

You rock man. So, I think I can read gps although I'm admittedly new to monitoring real-time sensor data and will be learning that as I go. I did see that segment of OBDWIZ the last time I had it connected.

If I understand what you said above, I would be looking for gps values out of line with what you stated. If I see values out of range, and there are no vacuum or exhaust leaks, then it would be logical to conclude that the MAF may be bad, yet not bad enough to trip the 101 code?

350350

Quick update- 171/174 I believe to be fixed. After removing all the lines, I found that the elbow that connects the PCV to the front of the intake had a split on the under side which I couldn't see from the top. Like busted completely open. So that was a massive vacuum leak that I missed with my visual inspection. Only about 200 miles on it since then so not entirely sure yet.

400 still hasn't come back. Wildly speculating that, because the vacuum port that powers the EGR solenoid is right next to that PCV leak, possibly the vacuum wasn't strong enough to operate it properly?

Will report back in 1000 miles or so in any case...