Problems underload would always lead me to put a pressure port adapter in the o2 bung, just to check the backpressure of the exhaust system. I don't know if you have a cat or not, but I have seen dual wall pipes collapsed too, but its not as common. I use my vacuum gauge as its reads a small amount of pressure. The adapter fitting can be made or the tool trucks have them.
Cats are long gone on this truck. I'll try the TPS idea next when I have time.
I guess one thing I could use some opinions on is what that video I posted proves. Could a vacuum leak still be a problem? What else could be a culprit - ECM, injectors, ????
It could be almost anything. You are there, we get to look at videos and read data that you interpreted. The gauge was blurry but I think it shows you have good vacuum at idle and it does drop when you open the throttle. Sometimes when I was stuck on a car I would recommend fixing what you know is wrong. With all that's disconnected you had better be throwing a handful of codes. If not then thats a big problem. Hook it all backup, clear memory and see if no codes show up. The factory gm manuals are a very good start. They have no code drivability charts, a lot of them end with "replace with known good.... and retest. At least it gives you a few Ideas before throwing your money at it. In your case you are not doing yourself any favors with the o2 sensor not hooked up. Will it fix the problem? Who knows but at least you will get to see if the fuel mixture goes rich or lean during its problem and the computer will get that info too. With headers, unless they are shorty with an o2 bung, you probably need a three wire heated o2 anyway.Originally Posted by GetItBilly
Last edited by donf; 11-01-2018 at 04:22 AM.
Your video appears to show vacuum readings changing along with engine rpm. To me it proves that your initial description isn't too far from the symptoms on video. The engine could be lean or rich so I'm suggesting you to enrichen the mixture. If it runs better the solution may be to find out why the engine's lean. That would also tie in with why the engine appears to run better if the MAP is unplugged.I guess one thing I could use some opinions on is what that video I posted proves. Could a vacuum leak still be a problem? What else could be a culprit - ECM, injectors, ????
This is a tough way to diagnose because you're filtering what you're telling us. Your initial description is "stumbles and bumbles under load." A stumble is a specific condition relating to how the engine runs. A bumble is a cartoon character. All we can do is guess at that you're trying to describe. Then you describe a condition that you'd like to solve. But the description sounds like mostly normal operation. Except you see a problem with it. We're left guessing again. And then you post a video as if it's conclusive. We haven't been there watching all the steps you've gone through in this fight. So your video shows that vacuum fluctuates and the MAP sensor readings fluctuate as well which again seems like normal operation.
Here's the deal. If you're only trying to gain insight about how the engine and ecm work together, then we can help. The ecm will vary timing based on load, rpm, coolant temperature, and knock signal. More rpm means more advance. More vacuum means less load means more advance. Colder engines get more advance. Knocking engines get less.
If you want assistance diagnosing the problem we can help with that also. But you need to help us understand what's happening because we aren't there with you. How did the problem begin? Did it come on suddenly? Are there times when it's better or worse? What does the underhood sticker say about setting timing? Is the engine stock? You mention a vacuum leak... did you have one previously? What did you do to fix it?
Sorry, either I'm totally missing something or I didn't explain what's happening in the video clearly. What I intended was to ask if I can focus on investigating a (hopefully) short list of specific things given the behavior and readings in the video in combination with items I've already done listed in my 1st post, like dist, plugs, wires, timing at 0, etc.
After 1:04 in the video, when the MAP is unplugged, my intention was to show via the vacuum gauge that the engine seems to be mechanically healthy per the classic tests of how the gauge's needle should react under various conditions. By the way, I do see vacuum increase a bit as steady rpms increase, which I believe indicates exhaust is flowing well.
At the start of the video, with the MAP plugged in and feeding the ECM, I intended the video to show, first, that the MAP sensor voltage readings look correct. Second, between :05 and :024, I am not manually blipping the throttle open and closed. I am doing my best to hold the throttle open at roughly 2000 - 2100 RPMs. This is my "stumble and bumble" :). It does the same thing under load in gear. In the video, RPMs are cycling/dropping from ~2100 to ~600-700. I don't think I can do an aggregated knock count with my scanner, but it never reports a knock. Coolant temp is being reported correctly via OBD.
So what I was hoping you could help me learn is what the ECM is trying to do here. I learned it uses RPM and MAP output to set timing. With MAP unplugged, it uses a default fuel map. RPM and MAP readings look correct via OBD scanner.
This is a new-to-me-truck - I have no standard or history to compare how it's running with. Engine might not be stock. When it first got delivered, timing was set at 8 degrees advanced, maybe even a little more. I guess technically it ran better than it does now, but still not really good at all, and idle RPM was so high it was a pretty brave move to actually put it in gear.
Last edited by GetItBilly; 11-01-2018 at 08:20 AM.
One tough aspect about diagnosing problems in a responsive system is trying to separate problems caused by the response from problems causing the response. I believe some of what you are looking at are problems caused by the response. The timing will vary as MAP varies. This can cause a cycle of hunting rpm. Timing further varies with RPM which can widen the range of the response. There's a chance that the timing is not the cause of the problem.
I am an "old fashioned" mechanic and sometimes I prefer to reduce the computer's ability to affect the engine and reduce complication. If I suspected the timing variations were masking a problem or causing further problems I would disconnect the tan/black wire to lock the timing in a fixed position. That would allow me to use a timing light to set the distributor where I want at a particular RPM then focus on the rest of the engine. I have a dial type timing light so I can read the timing setting well past the timing tab. I would set timing to an appropriate value to see if the engine runs smoothly and provides a steady vacuum reading.
Disconnecting the MAP sensor is not usually one of my options. With both fuel and spark controlled by MAP it really changes the computer's response. The fuel calculation is based on throttle position, coolant temp, and the VE map. It can be way off compared to the reality.
One issue that you can check is the base timing setting. On many of the early trucks it was six degrees. Setting timing six degrees retarded would drastically alter how the engine ran.
New ECM did not change behavior, it was worth a shot for $15.
With tan/black wire disconnected, max engine vac was achieved around 10-12 degrees advanced. But, per info on the truck's shroud, base timing should be set at 0 degrees.
Double checked the injector spray pattern - nice cone shape with no big drops. I see the amount of spray cycle up and down with the rpm drops shown in my video.
One thing I wanted to ask is if an o2 code should be triggered while in Park or if it needs to be in gear. With no o2 sensor plugged into the connector, I think I should expect code 13 to be triggered under these circumstances:
-engine running at least 40 seconds and
-no code 21 or 22 (TPS errors) present, and
-coolant temperature is at least 118 deg F (48 deg C), and
-O2 sensor voltage not fluctuating (i.e. steady between 350 and 557 mV), and
-TPS signal indicates above idle (over 6%), and
-all the above conditions met for more than 8 seconds
My truck throws codes for everything else as far as unplugging the timing wire, MAP, etc. Does that mean my prom has probably been messed with?
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