If your IAC system is not working properly and you have chronic low idle, you might be running rough at least in part due to the low idle rpm alone. Does it run any smoother if you bump the idle up 100-200 rpm?

George

 

WEDNESDAY: Thanks George, I have been thinking here, also trying some more stuff. And I'm just running in circles, I was chasing P0507, as in too much air entering the engine, I turned the stop screw out until IAC counts were in spec and now it has negative fuel trims? I wish I knew if they were negative when it was setting P0507? I would think they would have been positive? To follow this, I was watching scannerdanners video (linked below) and got the idea where he said "what does a vacuum leak do on a speed density engine, not much", so I went out and unplugged the MAF to see if it would run any better (meaning vacuum leak or MAF reading bad) and it actually ran a bit worse, so what could be going on here, hmm. I have heard or read that when engines default to speed density they usually run a little richer as programmed in the computer, this could be why it ran rougher as I'm already having a problem with a rich condition. Also, it runs a bit better when cold and idling a bit higher, but you can still feel it's rough a bit.
THURSDAY: So today curiosity killed the cat and I wanted to see my fuel trims actually working, of course to do this I just pulled the fresh air intake hose off and created a vacuum leak. I watched as the fuel trims went way positive signaling a lean condition. Then connected it back up and the fuel trims immediately followed by going back rich. I have a video below, it was interesting to me nonetheless to see the fuel trims working in real-time. Now I've been wondering here, would the fact my throttle blade is more closed than the factory have an effect on fuel trims? I realize the computer makes its calculations based on the MAF, but I would also think it makes its calculations based on how much the throttle blade was initially open for how much air is actually entering the engine? I think this would have probably been calculated at the factory and put into the computer? I have been thinking about this, more closed throttle blade = less air than expected = rich condition? Let me know guys, I'm not totally sure how the IAC would play a role in this. Also to note in the video when I made a vacuum leak the IAC counts stayed about the same it was only the fuel trims making the adjustment to keep the engine running, I would have thought the IAC counts would go to 0 trying to limit the amount of air? I will be putting in the new dist and fixing the exhaust leak next as previously said, probably on Friday. Assuming this makes no change I might look at putting the stop screw back to factory and seeing my fuel trims then. If this doesn't work I will be looking at blocking off the EVAP and PCV on the intake manifold then try starting it, I have seen while monitoring the fuel trims, whatever the problem is, it's affecting both banks equally. You would think a sticking injector (really the only possibility for a fault in the fuel delivery system now as it holds pressure meaning no leaks anywhere) would affect only one bank? As it would only be one injector? Hmm, hopefully I can find something with this testing mentioned as after that I'm running out of things to check.
FRIDAY: So I blocked the PCV valve today and saw no change, fuel trims stayed around -8, and plugging it back in didn't affect anything either. Still the rough idle in both cases.
SATURDAY: Today I installed the billet aluminum distributor, changed all the internals to ACDelco. A bit frustrating, I set the engine to TDC on #1 with the distributor still in, so crank marks lined up and dist lined up. Then trying to put the new one in I swear it just didn't line up the same, eventually I just said screw it and started it up so I could see cam retard, which was only -6 or so and I turned the dist so it now is -1 degree. Good enough for me right now, however still did not change anything with the idle. After this I tried blocking off the EVAP solenoid and got some mixed results, fuel trims stayed around -3.9 on both banks, with the short term looking happy. Still ran rough. But the thing is it was not warmed up fully, after I plugged the EVAP hose back in and ran it for a few more minutes (not instantly) the long term started climbing more negative. I'm going to see tomorrow if this happens every time it's cold or what, and yes, the exhaust leak will be fixed too. I'm running out of things to check here guys, what do you think here? I know this is a long post as it's over a few days but I just kept finding nothing, not very inspiring.

 

I am sure that Les will have some thoughts so I will just comment on the fuel trim related stuff.

First, an engine of this age will not have zero fuel trims. Totals (STFT+LTFT) below 10 absolute value is considered OK. Closer to zero is always better but not reasonable at 100's of thousands of miles and decades of time. Your doing good work here to understand everything like creating vacuum leaks and watching fuel trims but just make sure that your not chasing ghosts, especially before you fix that exhaust leak. Looking for other air leaks is reasonable

Yes, a single faulty fuel injector will affect that bank. There are a couple of tests for injectors if you are so inclined.

Do you mean evap vent solenoid (at the end of the venting chain near the fuel tank) or purge solenoid near the engine? Are we looking for a vacuum leak?

I have never touched the idle screw but I would think that turning it at idle would cause the IAC counts to change as the computer compensates. What happens as you turn that screw first with the IAC connected and then again with it disconnected? Did you guys ever conclude if the IAC system was working properly, I dont remember? TPS is one of the inputs for the fuel cell base case calculation from which fuel trims are derived as the O2 request for fuel delivery is compared to the fuel cell look up value. I am not sure if the small changes in TPS position from turning the idle screw is enough to significantly change fuel trims.

George

 

Thanks George, so I have read many times +-10 total fuel trims are within spec. However, I will check it at times and both banks will be pinned long term at -13% while just idling in park, I'll even rev it up a bit to shake the fuel trims up and they will end up in the same spot when it's back idling. Making me think there's a direct problem between it running rich, and running rough. I would also think on a freshly rebuilt engine the fuel trims would stay very close to 0 too? My thought process behind blocking the EVAP off was to put any thoughts to rest that too much "rich" air was going into the engine from the EVAP system, same thought process for the PCV valve. I pulled the hose off the purge solenoid at the intake, then put some electrical tape around it and started it up, same thing with the pcv valve. I also wanted to block these off as that's all of the intake manifold connections I hadn't blocked off yet. I'm starting to think it may be a sensor here, I will do some more looking and hopefully fix the exhaust leak today.

 

13% is starting to get worthy of chasing, I saw your video thumbnail and they were under 10.. Get that exhaust leak fixed and then we can chase rich condition problems if they still exist.

Where are you guys on that original failed dynamic compression test and all the compression tester issues? IOW, have you ruled out breathing and volumetric efficiency problems?


George

 

Exactly George, and the fact the long term were down at -3.9% with the EVAP solenoid disconnected is interesting. I will say when the fuel trims are -13% will be checking after a drive most times thoroughly warmed up. I'm wondering, when do rich codes start coming then? -13% must be getting close I would think? I will be testing more today to see if the fuel trims change with temp or what, also after and before a drive. So with the compression testing, I got a new OTC tester and had no more problems with the testing. All the cylinders are about the same as each other, I tested cylinder 1 thoroughly again and concluded it was indeed inaccurate in the first test, it's just like all the other cylinders. About the IAC, the courts were 0 when it was setting P0507, after checking for vacuum leaks I adjusted the stop screw out until I had around the same counts as Les at idle. So I did watch it adjust in real-time, seems to be working as intended. It's weird though, that the IAC didn't really respond to the vacuum leak I created?

 

I believe we set a code at trims above 25%

George

 

Personally, I would like to see restricted exhaust/converters eliminated as a possible problem. Things can happen in the pipes and muffler in addition to converters. If MAF sensor wont reach the value it should upon shift point, the problem could be either a MAF sensor or a volumetric efficiency problem. I had hoped to get the volumetric efficiency info off the running compression testing, but I have a hard time believing the snap tests are really being done full throttle from 1200 to 2500 rpm given the results. I suppose the MAF sensor at full throttle shift may be a better way to test volumetric efficiency, even though it is an overall (not individual cylinder). If MAF test is good then we can rule out exhaust restrictions, know that the strange snap test readings are not real, and that the MAF sensor is good. If MAF test fails, then either the MAF is bad or something is affecting the volumetric efficiency. I keep wondering why the exhaust decided to start leaking where and when it did.

 

Thanks George and Les. So before I get into things here, I put a new gasket on the exhaust yesterday, no more leak, and it still runs rough as hell. Les I was actually thinking the same as you. I was looking at back pressure testers today, wow they are quite pricy for a nice one, like the OTC one is $200! I'm not sure what you mean about the MAF sensor test for volumetric efficiency and exhaust restrictions, if you could explain it a little more I will definitely try and do it. I may even end up getting a back pressure tester, let me know if you guys think it would be worth the test, I will not get the OTC one if I do get one, maybe the OEMtools one for around $70. Snap tests were just that, full throttle for a brief second, what is wrong with the results? The snap was consistently 30psi higher than idle for each cylinder, while cold it would be 90 idle 120 snap, hot I was getting 60 idle around 90 snap. Now I also read this over on the s10forums,

"Say the engine is running at 18 inches vacuum. Atmospheric pressure is about 30 inches, so the difference (30 inches - 18 inches = 12 inches) is what the engine is sucking in. 12 inches mercury is equivalent to about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you should get 6 x 8 = 48 psi pressure if all the air makes it into the cylinder and then gets pushed out. So your idle reading on running compression is about 50 psi.
When you snap the throttle, the manifold vacuum drops, so the absolute air pressure going into the cylinder increases.
In fact, you can do running compression tests at various constant manifold vacuum readings (by brake-torqueing the engine momentarily), and the running compression should roughly correspond to the manifold vacuum. For example, at 10 inches vacuum, engine should be breathing in about 10 psi air pressure, so you should see a running compression reading of about 80 psi (at 8 to 1 compression ratio)."

I think I will take a reading of the MAP and do some calculations to see exactly where the running test should have been, I read online the 4.3 has a compression ratio of 9.2:1 so I will use that for the calculations. More on the exhaust possibility now, along with the rough idle, I have noticed she likes to run a little hot at specific times. Now it will pretty much only run hot when I'm giving it the juice for a bit, like going fast up a hill with the A/C on using lots of power. Let me tell you guys a story, this keeps happening as well, same place different time has happened a few times. So in the place I go off roading right near my house it's pretty much a big hill climb for the first 10km, everytime I get to where I turn off and it flattens out, it's pegged running hot, and stays there. What saves me every time is the fact on the flats right after I turn there are many big puddles that I fly through and that always seems to get the temps back to normal. But the temp has been around the 3/4 mark when I've hit the puddles before and I'm honestly afraid if those puddles dry up I'm going to have one hell of a day out there. Adding onto this I live at the top of a big hill, coming home from work recently I have noticed it running hot right after giving it some juice to get up the hill (it's about a 5 min drive up the hill going 70km/h). Today I even got a video of it, posted below (I'm going to be honest, the video isn't that great as I just pulled out my phone quickly to record as I was driving, I saw the temps were hot and I wanted to capture it also the tablet wasn't running at the time), sure enough by the time I got home the temp already flattened back out to normal. Now I am running straight water still, it is around 35 degrees here (very hot), but this in the end shouldn't make a difference right? I mean I still have lots of coolant(water) in there, the engine should run at 100 degrees, even a little more is still fine as water will boil a bit past 100 degrees since it will be under pressure, right? I plan on switching to real coolant in the next bit here anyway, although I think that will have no effect on this temperature issue. It will keep the normal temperature at all times excluding when I'm being demanding of the engine for a decent duration. What you said about wondering why the exhaust blew apart got me thinking a bit Les, we both know it blew apart when I had a leaking spider in there, but it has had P0420 ever since it blew apart at the flange, something must have happened in there for it to blow the whole gasket out, and whatever happened, P0420 has been set ever since. My other thought is to have a quick look at the coil in the dark, I don't expect to find anything as misfire counts are fine but it won't hurt to look for any arching that might be there, could a "weak" coil be a possibility? It seems more realistic that a coil would either work as expected or not work at all but let me know guys. I'm starting to place my bets on the exhaust here, I can't believe I was thinking about my cat all day looking at different posts on the forum then I come here (I missed the notification you replied somehow) and you said something about it too LOL.

 

The MAF sensor measures volume of air going into the engine. % Volumetric efficiency is defined as volume of air actually passing, divided by the engine displacement times 100. This is about 80% on a stock engine. If exhaust is restricted, volume of air through engine is reduced and MAF will read lower than normal as a result. A normal MAF reading at wide open throttle 1) indicates normal volumetric efficiency and 2) eliminates the possibility of restricted exhaust. (an abnormal MAF reading is not as clear cut, but indicates a problem with either 1) general volumetric efficiency/misfire or 2) the MAF sensor. Just read the MAF sensor at the moment of 1-2 full throttle shift and note the max RPM.

I think your snap tests are way low for 175psi cranking compression across the board. So I must assume that you are not getting full throttle (vacuum = zero) during the test, or not doing it long enough to build up the pressure. Why I suggested we get at general volumetric efficiency another way that takes out these possibilities.

You can play doing those calcs, but please be advised that actual numbers depend on dynamic (effective) compression ratio due to valve timing and that is engine/camshaft specific. I'm sure that engineers can calculate this stuff, so you are definitely thinking and I applaud you for that! But I believe it is much more complicated than what was presented in the S10 forum. If you are interested read up on dynamic compression ratio calculations from an engineering standpoint.

Straight water boils at 100C, but you have it under pressure. Radiator caps are about 16 psi. 50% antifreeze raises the boiling point even further. Modern cars run around 95-100C+. I really didn't see in your video that I thought it was overheating. But you may want to put antifreeze in it for water pump lubrication and corrosion inhibition.