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The ground path from engine block to battery is important. I recommend O.E. cable or a quality factory made piece. Also be sure the connections at the back of the block are clean and sound.
The AFM mixture adjustment screw is an unmetered (false) air bypass. Proportionally, it will have more effect at idle but is false air throughout the range.
I would take some manifold vacuum readings but am confident there will be false air sources around the AOS and idle valve circuits.
You may want to stop at a dyno facility or use a wideband O2 sensor and gauge to see if you're running lean. The ethanol blend fuel is really different than what the 944 is used to from the 80's. I adjusted the fuel quality switch on the DME to richen the mixture (add more fuel) on my '84 944 which made it run much better.
Quick question is your car hard to start?
Do you have to give it gas to get it to start?
Have you messed with the mixture screw on the afm?
Or prob the previous owner has and adjusted it if they did the O ring is prob shot and allowing air to leak by had an issue similar to yours
the screw would vibrate out because the o ring was shot making the car hard to start when warm
Nox only forms when your Cylinder temps go above 2500 deg F a lean fuel mixture will burn hotter
Ok, so I just got my fuel injectors back from South Bay Fuel Injectors (thanks for the recommendation, T&T Racing). Great service and quick turnaround. Before and after flow measurements are attached below. It looks like they actually may have not been in too bad of shape after all? Either way it was worth it just know. T&T Racing, would you still recommend a second fuel filter?
The ground path from engine block to battery is important...Also be sure the connections at the back of the block are clean and sound.
Gage, that makes a lot of sense. I replaced the battery ground cable the other day on your recommendation. Thanks for the tip! Is there another return path from the block that I should inspect and potentially replace. Where can that be found?
There are three hoses, (idle air in, out and AOS) mostly concealed by the intake manifold, that are nearly guaranteed to be leaking false air to the mixture and crankcase.
With the fuel rail and injectors removed I was finally able to get the intake manifold off, and gave it a good clean inside and out. I also have fresh gaskets to go on the manifold when I place it back in. Gage, as you predicted, it looks like an AOS line had at least a visible crack in it. Some of the hoses also did not seem to be seated all that well. With the opportunity available I'm doing my best to replace all of these lines under the manifold this week, and ensure they are secured tightly. Should I also replace seals on the AOS while they're easy to get at?
The AFM mixture adjustment screw is an unmetered (false) air bypass. Proportionally, it will have more effect at idle but is false air throughout the range.
That makes a lot of sense, unmetered air is unmetered air regardless of the % that it contributes. I checked the screw and it looks like it is 1.5 turns from the bottom. I'm not sure what the expected number should be?
The ethanol blend fuel is really different than what the 944 is used to from the 80's. I adjusted the fuel quality switch on the DME to richen the mixture (add more fuel) on my '84 944 which made it run much better.
RTK, this makes a lot of sense. If I exhaust all of my efforts to remove false air leaks, this definitely feels like the next step? How will I know which position to try?
Dwizzle, thankfully it looks like the car starts pretty consistently without fuss (knock on wood), without giving it any fuel. I haven't messed with the mixture screw myself, but I've got no information from the PO.
Or prob the previous owner has and adjusted it if they did the O ring is prob shot and allowing air to leak by
Interesting. I wasn't aware of this O-Ring. Seems worth replacing just in case and returning the screw to its current setting after?
In general with the manifold off is there anything else worth getting at or checking before I button it all back up and take it back in for another go at Smog?
No need to add second fuel filter, injectors are go to go.
I would replace AOS seals, cheap, to discount any air leakage on the top o-ring.
Here is the link for FQS https://members.rennlist.com/951_racerx/FQS-adjust.html
There are two options available
1. Do all the maintenance on AOS and potential leaking sources of unmetered air, verify FQS and reset if not position 1, retest
2. If fail, Do 1, and set FQS to highest + fuel, retest.
If passes only on option 2, reset FQS to next lower + fuel and then reset to option 2 for next smog test
GL with path forward
Thanks T&T Racing! That sounds like a great course of action. I'll definitely be doing that.
I finally bolted everything back together (new manifold gaskets, cleaned fuel injectors, AOS seals, various vacuum lines replaced, intake manifold cleaned, Venturi tube resealed). Maybe it's all in my head, but I think it's driving noticeably better and feels more consistent! It also seems less rough at Idle. That being said it may have developed new symptoms that were not there before this last round of work...I'm not sure if that's the result of catching leaks and throwing off the tune that was compensating for the lean condition, or because I've potentially made a mistake. I'd appreciate thoughts before I take it in for another (costly) smog test:
1. The steady state idle is pretty high. When starting the car, it revs high while starting (as before) and then drops down to about 900 to idle (so far so good), but then given a few seconds the idle rpm slowly starts to rise, until it bounces about 1100 or 1200 and stays there for the rest of the drive. Is this normal behavior? For the high Idle should I attempt to tune the bypass air screws on the MAF and TB? One other thing about starting, a couple of times it has started fine, reved high on the start, and then when it is returning down to idle, it shoots right past 900 and goes to zero, and dies (unviolently). A second attempt at restart usually works fine though. The connection to the MAF seems fine (I know that can cause it drop to zero after a start).
2. In the past whenever I've turned on the accessories, the fan wakes up immediately, even before starting (I presumed this was normal). As of late (after this round of repairs, but not immediately after, it worked fine for 5 or 6 starts) this has suddenly stopped. More concerning to me is that the temperature on the gauge does seem to gradually climb when sitting, and can get over to the third tick, if given the opportunity. To me that seems like the radiator fan switch is having trouble? The AC **** still blows air when turned (from my novice understanding that is the same fan?).
If videos or audio help I'm happy to record and upload. Thanks again for the help, guys!
Check Clarks garage for setting the idle with the ISV also check the idle switch that it's firmly seated when closed so you aren't getting false throttle movements at idle.
I think the idle switch also tells the fans when to come on.
Thanks T&T Racing! That sounds like a great course of action. I'll definitely be doing that.
I finally bolted everything back together (new manifold gaskets, cleaned fuel injectors, AOS seals, various vacuum lines replaced, intake manifold cleaned, Venturi tube resealed). Maybe it's all in my head, but I think it's driving noticeably better and feels more consistent! It also seems less rough at Idle. That being said it may have developed new symptoms that were not there before this last round of work...I'm not sure if that's the result of catching leaks and throwing off the tune that was compensating for the lean condition, or because I've potentially made a mistake. I'd appreciate thoughts before I take it in for another (costly) smog test:
1. The steady state idle is pretty high. When starting the car, it revs high while starting (as before) and then drops down to about 900 to idle (so far so good), but then given a few seconds the idle rpm slowly starts to rise, until it bounces about 1100 or 1200 and stays there for the rest of the drive. Is this normal behavior? For the high Idle should I attempt to tune the bypass air screws on the MAF and TB? One other thing about starting, a couple of times it has started fine, reved high on the start, and then when it is returning down to idle, it shoots right past 900 and goes to zero, and dies (unviolently). A second attempt at restart usually works fine though. The connection to the MAF seems fine (I know that can cause it drop to zero after a start).
2. In the past whenever I've turned on the accessories, the fan wakes up immediately, even before starting (I presumed this was normal). As of late (after this round of repairs, but not immediately after, it worked fine for 5 or 6 starts) this has suddenly stopped. More concerning to me is that the temperature on the gauge does seem to gradually climb when sitting, and can get over to the third tick, if given the opportunity. To me that seems like the radiator fan switch is having trouble? The AC **** still blows air when turned (from my novice understanding that is the same fan?).
If videos or audio help I'm happy to record and upload. Thanks again for the help, guys!
before you spend a crudload of money on cooling parts that you may or may not need, check out the temp gauge potentiometer test on clarks garage. That was the last test i did in diagnosing my “overheating” issues and come to find out the gauge read about 20*c higher than the actual engine temp.
Finally some good news! Just passed Smog today (numbers below)! Thanks to everyone for all of the input on solving this problem. I would have been hosed without this thread. I tuned the idle and bypass on the MAF to get the idle down and richen up the mixture a bit. Looks like that, with all of the other changes, was finally enough to not just push me over, but get some pretty decent numbers
before you spend a crudload of money on cooling parts that you may or may not need, check out the temp gauge potentiometer test on clarks garage. That was the last test i did in diagnosing my “overheating” issues and come to find out the gauge read about 20*c higher than the actual engine temp.
That being said I'm still overheating, even after a new (lower temp) fan switch and repeated venting. Neversinksmith, that's a really interesting thought. I'll give that a look and report back. It would be nice to be just a perceived problem...
02-16-2021, 12:11 PM
