Major hesitation at wide open throttle
02-18-2016, 01:34 AM
#31
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Start off with the easy stuff. Relay-relay-relay. Jumper the relay and see if the fuel pump sounds OK. Plug in a fuel pressure gauge. Do the fuel delivery test. By now you have run through the tests that narrow the problem to or eliminate the problem from the fuel system. From that you decide what's next.
Troubleshooting is the science of narrowing down possibilities in a very systematic method. Do these fuel system tests and cut the possibilities list in half.
Troubleshooting is the science of narrowing down possibilities in a very systematic method. Do these fuel system tests and cut the possibilities list in half.
02-18-2016, 02:07 AM
#32
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Not necessarily. What you have here is a a major power loss at higher RPM under load. Power requires air, fuel, spark-- one is missing under high load, high-ish RPM. I think "air" is the ONLY one that can be eliminated, assuming the airfilter is not full of leaves and the throttle cable is not broken. Which leaves fuel or spark.
"Fuel" requires pressure in the rails and a signal to open the injectors. The pressure is fuel pump, injectors involves a bunch of electronics-- MAF, LH, various sensors, ground connections and harnesses. However, none of that electronics stuff much cares about load or RPMs, so I would put that stuff pretty far down the list-- besides which, some it has already been tested or replaced.
So here's the deal with fuel: You've got a pump in the back which continually pumps fuel to the rails, through the fuel-regulator and back to the tank-- a big loop. The flow rate is constant-- whatever the pump can deliver against a 55 psi head pressure-- the fuel regulator. The spec is 2.5 liters a minute, around and around. And whatever fuel the engine needs is taken from that flow-- very little at idle, more at higher RPM and more at WOT, and max RPM at WOT takes almost all of it.
Now suppose your pump has gone poopy, or the filter is clogged, or the inlet screen has gotten plugged with crap. It won't be 2.5L/min, maybe only 1.0L/min.
What happens when you get to 4000 RPM and max throttle? The injectors are now squirting more fuel than the pump can put out, the pressure drops, the mixture goes very lean, and the engine quits making power.
So yes, this scenario definitely fits fuel pump, filter or screen problems. There is a simple test in the WSM for measuring fuel flow at the tank return line, do that before spending more of anyone's time speculating further. Disconnect the return hose form the top of the tank, add an extension and run it into a gallon jug, jumper the fuel-pump for 30 seconds, and measure how much fuel got pumped. For an S4, 1250cc in 30 seconds (2.5L/min). While you are there, pop the sender out and eyeball the screen-- any yuckus in the tank? Change filter, test again, then change pump if needed. They don't last forever and the "modern" fuels are not kind to original pumps.
Spark: You would think that an ignition problem would show up with any load or RPM, right? Not so, as pointed out above this just isn't the case. The reason is simple: To make a spark you need enough voltage to jump the spark-plug gap? And for normal room air that isn't much, maybe 1000 volts (1KV).
Here's the trick question: What is the air pressure at max compression? (15 psi x 10:1 compression). How much voltage is required to jump a gap at 150 psi air pressure? Well over 10KV. Any broken insulation suddenly becomes a very attractive alternative path, and no spark.
So a high-load miss could definitely be an ignition issue. The "firefly" test will find a bad case of broken-down insulation, but you can't do that under load. The best option (IMO) is careful inspection. Or just replace the wires if age and condition are uncertain. They are "consumables", just like fuel pumps...
Coils, likewise, can break down internally-- if the insulating "goo" inside starts leaking out then the spark can jump to ground inside the case, and no spark. This would effect all cylinders on one bank.
The nature of the "miss" can tell you lots: At the transition, when it starts acting up, does it feel or sound like all cylinders are cutting out for a moment and then coming back, a "stuttering" or "lurching"?? (That would be fuel pump or filter). Or only one cylinder cutting out, clearly missing but still pulling? (Plug wire). Or does it feel and sound like if is running on half the cylinders, never cutting out completely but running with half an engine sort of like a 944? (That would be a coil or coil wire).
You need to spend some time testing, there are still too many possibilities.
"Fuel" requires pressure in the rails and a signal to open the injectors. The pressure is fuel pump, injectors involves a bunch of electronics-- MAF, LH, various sensors, ground connections and harnesses. However, none of that electronics stuff much cares about load or RPMs, so I would put that stuff pretty far down the list-- besides which, some it has already been tested or replaced.
So here's the deal with fuel: You've got a pump in the back which continually pumps fuel to the rails, through the fuel-regulator and back to the tank-- a big loop. The flow rate is constant-- whatever the pump can deliver against a 55 psi head pressure-- the fuel regulator. The spec is 2.5 liters a minute, around and around. And whatever fuel the engine needs is taken from that flow-- very little at idle, more at higher RPM and more at WOT, and max RPM at WOT takes almost all of it.
Now suppose your pump has gone poopy, or the filter is clogged, or the inlet screen has gotten plugged with crap. It won't be 2.5L/min, maybe only 1.0L/min.
What happens when you get to 4000 RPM and max throttle? The injectors are now squirting more fuel than the pump can put out, the pressure drops, the mixture goes very lean, and the engine quits making power.
So yes, this scenario definitely fits fuel pump, filter or screen problems. There is a simple test in the WSM for measuring fuel flow at the tank return line, do that before spending more of anyone's time speculating further. Disconnect the return hose form the top of the tank, add an extension and run it into a gallon jug, jumper the fuel-pump for 30 seconds, and measure how much fuel got pumped. For an S4, 1250cc in 30 seconds (2.5L/min). While you are there, pop the sender out and eyeball the screen-- any yuckus in the tank? Change filter, test again, then change pump if needed. They don't last forever and the "modern" fuels are not kind to original pumps.
Spark: You would think that an ignition problem would show up with any load or RPM, right? Not so, as pointed out above this just isn't the case. The reason is simple: To make a spark you need enough voltage to jump the spark-plug gap? And for normal room air that isn't much, maybe 1000 volts (1KV).
Here's the trick question: What is the air pressure at max compression? (15 psi x 10:1 compression). How much voltage is required to jump a gap at 150 psi air pressure? Well over 10KV. Any broken insulation suddenly becomes a very attractive alternative path, and no spark.
So a high-load miss could definitely be an ignition issue. The "firefly" test will find a bad case of broken-down insulation, but you can't do that under load. The best option (IMO) is careful inspection. Or just replace the wires if age and condition are uncertain. They are "consumables", just like fuel pumps...
Coils, likewise, can break down internally-- if the insulating "goo" inside starts leaking out then the spark can jump to ground inside the case, and no spark. This would effect all cylinders on one bank.
The nature of the "miss" can tell you lots: At the transition, when it starts acting up, does it feel or sound like all cylinders are cutting out for a moment and then coming back, a "stuttering" or "lurching"?? (That would be fuel pump or filter). Or only one cylinder cutting out, clearly missing but still pulling? (Plug wire). Or does it feel and sound like if is running on half the cylinders, never cutting out completely but running with half an engine sort of like a 944? (That would be a coil or coil wire).
You need to spend some time testing, there are still too many possibilities.
02-18-2016, 02:30 AM
#33
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This is a very helpful description of things. I think your 3rd option above is the most accurate description, so I'm leaning toward coils. It is going to the shop next week for a work-up and I will point them in that direction. I have had a fuel pump issue way back when on my 944, and it was more like your option 1, a complete fuel starvation that was intermittent, producing a very powerful lurching accompanied by a weird sucking noise. My car seems more like it is running on 4 cylinders alternating with 8, producing a power loss and bucking that is intermittent but not a complete lurching with zero engine power.
As Dr Bob quite rightly says-process of elimination is imperative here and an excellent analytical post by Jim. The ignition leads could also be failing but in my experience that tends to be more a subtle loss of power initially that still needs to be ruled out.
Rgds
Fred
02-18-2016, 10:01 AM
#34
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Very easy to pull the coil to distributor lead and examine both ends, especially the coil end, and the coil itself. Corrosion of the coil end of the lead, and in the coil itself is common, and so are almost microscopic cracks in the coil "nose" which can sometimes be identified by the smudgy carbon trace on the inside and/or outside of said "nose".
02-18-2016, 10:17 AM
#35
02-18-2016, 12:26 PM
#36
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it wont miss in neutral with this problem.. it wont even miss at part throttle acceleration. it seemed to only be noticeable when WOT was used and at the higher RPM ranges. however, you could see(hear) arc'ing near the coil wires on the coil. thats why i suggested a night time observation of the coils just to be sure its not something simple. otherwise..... see fuel problem possibilities as been described by others here.
02-18-2016, 08:20 PM
#37
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Your PO replaced the ignition wires with complete new Beru set in the year or so before you bought the car. Rolf subscribed to my "do everything you can in the comfort of your garage." philosophy, to avoid stupid stuff whenever possible. He started of buying a set of MR Beru wires to use with his old connectors, but his shop convinced him hat their time to assemble was more than the upgrade cost to the assembled set.
Based on your updated/enhanced symptom description, do as the others suggest and remove/clean/reinstall both ignition coil wires. Make sure the two ends on each are actually still screwed into the wire, and use an Ohm meter to confirm that there's little or no resistance between the ends. Discoloration of the brass ends, either arc-flash points or a green color are worth investigating. If there's anything amiss, grab an inspection mirror and look into the end of the coil for similar contamination or weld/spattermarkings from arcing.
------
In a slight bow to my old-school ways, I've made small "current trasformers" out of a few short short lengths of stiff copper wire. Start off with three pieces maybe four inches long, identical length. Wrap the copper wire around the coil wire with 4 tight turns, so each 'wrap' is touching the next. Do the same on the other coil wire, making sure you wrap in the same direction on both. Pigtail end comes off the wrap at a right angle to the coil wire. Third one goes onto the #1 plug wire. I then connect my handy ancient two-channel oscilloscope to the coil wire transformers, and the #1 cylinder transformer to the 'sync' input on the 'scope. Overlaping patterns on the 'scope show you amplitude of each ignition pulse, with the spikes for each showing up in cylinder firing order starting with #1 on the left. 4 Spikes from each coil, then they repeat again from the left.
You can see on the spikes where, in a perfect world, they would all be exactly the same height (voltage) and width (duration). A wire that's leaking to ground will show up with a different height (lower). If a coil is weak, all the cylinders fired by that coil will be lower.
If you suspect that a transistor "trigger" unit on the front apron is getting weak, you can move the 'scope leads directly to the primary side of the coils. Pretty much independent of the high-voltage side, the two primary patterns should be identical; a weak one will show up instantly as the lower peaks.
After you rule out the fuel system... (passes pressure and flow/delivery tests):
For most, just swapping in a new pair of coils is the troubleshooting step after verifying the coil wires. Then the modules as next step. For either of these, be SURE to get the correct Genuine Bosch replacements. There are more than enough folks who have gone cheap on those and reported problems worse than they were originally chasing.
Another hint: A working oxygen sensor will indicate a lean condition for you (fuel delivery/pressure or tired MAF) when connected to a simple voltmeter. In closed-loop operation, you expect to see voltage relatively stable around ~0.4VDC (four hundred millivolts). A misfire (ignition-caused) doesn't change the sensor reading much if at all, so low voltage is usually a telltale of fuel delivery issues or air leaks. You'll want to watch it in service (still connected) as you drive and load the engine.
02-19-2016, 04:25 AM
#38
Burning Brakes
"Another hint: A working oxygen sensor will indicate a lean condition for you (fuel delivery/pressure or tired MAF) when connected to a simple voltmeter. In closed-loop operation, you expect to see voltage relatively stable around ~0.4VDC (four hundred millivolts). A misfire (ignition-caused) doesn't change the sensor reading much if at all, so low voltage is usually a telltale of fuel delivery issues or air leaks. You'll want to watch it in service (still connected) as you drive and load the engine."
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
02-19-2016, 06:57 PM
#39
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"Another hint: A working oxygen sensor will indicate a lean condition for you (fuel delivery/pressure or tired MAF) when connected to a simple voltmeter. In closed-loop operation, you expect to see voltage relatively stable around ~0.4VDC (four hundred millivolts). A misfire (ignition-caused) doesn't change the sensor reading much if at all, so low voltage is usually a telltale of fuel delivery issues or air leaks. You'll want to watch it in service (still connected) as you drive and load the engine."
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
if it goes full blue at WOT, and is missing while blue, its spark. if it goes in to the yellow or red, its fuel related.
the problem of just using a meter is that full lean to rich is only 1 volt.
02-19-2016, 07:17 PM
#40
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"Another hint: A working oxygen sensor will indicate a lean condition for you (fuel delivery/pressure or tired MAF) when connected to a simple voltmeter. In closed-loop operation, you expect to see voltage relatively stable around ~0.4VDC (four hundred millivolts). A misfire (ignition-caused) doesn't change the sensor reading much if at all, so low voltage is usually a telltale of fuel delivery issues or air leaks. You'll want to watch it in service (still connected) as you drive and load the engine."
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
Is there an easy way to connect a voltmeter so you can monitor as you drive?
Thanks
The max available span is something like 800 millvolts, and the signal moves around a lot as load and LH 'correction' vary. Digital meters are all the rage these days, but the sample and settling time are way too slow to get anything more than a ballpark reading. Old-school analog meter with reasonably high impedance is the common weapon of choice. Mine is that old-school Tek storage 'scope with millivolt plug-in, so I can plot the readings over a bit of time.
The fitted oxy sensors are amazingly narrow in their sensing and amazingly non-linear in their outputs. For any serious measurement-based diagnostics, a wide-band sensor with the matching interface box is a worthwhile investment, especially if you plan on watching a display as you drive. Colored lights are easier to comprehend at speed than flickering numbers or a squiggly trace on a 'scope.
For the OP's situation, the pass-fail filter from the fuel pressure & delivery test will instantly narrow the possibilities list.
02-20-2016, 02:24 AM
#41
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Well I've eliminated ignition wiring as a culprit, since it's all new. . . if it's coils, it's because they've broken down inside. Going to get it figured out next week, we will see if it's fueling or coils. I just drove 70 mph for several miles down the freeway with totally smooth response -- until you give it real gas. If you drive like a grandmother you'd never even suspect anything was wrong with the car. So we're going to eliminate fuel delivery as a possibility but it seems likely to be the coils.
02-20-2016, 02:37 AM
#42
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Based on your updated/enhanced symptom description, do as the others suggest and remove/clean/reinstall both ignition coil wires. Make sure the two ends on each are actually still screwed into the wire, and use an Ohm meter to confirm that there's little or no resistance between the ends. Discoloration of the brass ends, either arc-flash points or a green color are worth investigating. If there's anything amiss, grab an inspection mirror and look into the end of the coil for similar contamination or weld/spattermarkings from arcing.
I'm not seeing anything in the WSM.
02-20-2016, 03:30 AM
#43
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Just passing through....
The OP title sounded like a prius bashing thread, just lookin to jump on the bandwagon.
..you know, prius in the left lane...
Never mind.
The OP title sounded like a prius bashing thread, just lookin to jump on the bandwagon.
..you know, prius in the left lane...
Never mind.
02-20-2016, 04:46 AM
#44
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Grounds first. Best to change the negative battery strap. If it is internally corroded, as even California cars have experienced, or loose, yes it can indeed act perfectly until car is under load at speed and cause those symptoms. Don't discount this. A lot of current must be pulled at speed, under load. The battery ground strap can be partially corroded such that not enough current can pass through the strap, and it only be evident at certain times or vibration conditions. Been there, done that, back in 2010. Results basically kicked-off this whole mantra you hear about age-out of ground straps. Ensure the - strap on the battery, the little +red wire on the battery, and the engine strap under passenger side engine/fender are tight. I'd replace the strap on the battery though. Even Dr. Bob IIRC had a problem with his failing.
Then check coil ends for corrosion. On east coast where we have a lot of moisture, one of the first places to look is with a mirror and flashlight inside where the coil wires get plugged. Often there is whiteish corrosion that manifests during load to cut out.
Then inspect every inch of 'new' wires for seemingly minor nicks in insulation.
These cars are notorious for cutout under load due to a nicked wire, especially a coil wire, arcing-out under load.
There were changes made to coil wires to reduce the arc out possibility 88 or so IIRC from tech briefs. One method of testing for wire or coil wire arc-out is at night to open the hood, apply brakes, and load-up the engine while in drive, while a helper peers into engine compartment looking for blue spark arc outs. *Try not to get run-over.
Not sure what wire set is on your car. Is it the Behr official set or the non-Behr cheap set? The Behrs use screw on connections. The economy set is a nightmare as it uses a cheap-*** nail pin connection to pin each wire to each end. Trying to screw them on/off to see if they are tight enough will destroy them.
There are different coil wire ends and different coils depending on year. Do you have the right version to match your coils? The behr set I bought came with one set for the early s4 and one set for the later s4. Did they use the right ones? Do they firmly snap in place or just kinda sorta plug in like milk toast.
There are smaller ground wires on both passenger and drivers side coils, ignore them, they are for radio noise suppression.
Then check coil ends for corrosion. On east coast where we have a lot of moisture, one of the first places to look is with a mirror and flashlight inside where the coil wires get plugged. Often there is whiteish corrosion that manifests during load to cut out.
Then inspect every inch of 'new' wires for seemingly minor nicks in insulation.
These cars are notorious for cutout under load due to a nicked wire, especially a coil wire, arcing-out under load.
There were changes made to coil wires to reduce the arc out possibility 88 or so IIRC from tech briefs. One method of testing for wire or coil wire arc-out is at night to open the hood, apply brakes, and load-up the engine while in drive, while a helper peers into engine compartment looking for blue spark arc outs. *Try not to get run-over.
Not sure what wire set is on your car. Is it the Behr official set or the non-Behr cheap set? The Behrs use screw on connections. The economy set is a nightmare as it uses a cheap-*** nail pin connection to pin each wire to each end. Trying to screw them on/off to see if they are tight enough will destroy them.
There are different coil wire ends and different coils depending on year. Do you have the right version to match your coils? The behr set I bought came with one set for the early s4 and one set for the later s4. Did they use the right ones? Do they firmly snap in place or just kinda sorta plug in like milk toast.
There are smaller ground wires on both passenger and drivers side coils, ignore them, they are for radio noise suppression.
02-20-2016, 05:36 AM
#45
I struggled and struggled with a similar problem, then someone mentioned bad gas... That turned out to be my issue. It took a few tanks to fully clear it out, but has this issue emerged out of the blue recently, and have you gone through a tank of gas yet? I am amazed on how the simplest of things can cause the biggest problems