When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I bought the car yesterday with an issue... Engine chokes under partial and/or hard acceleration. After 4.5-5k rpm there's a drop in torque very noticeable and it's not delivering those 296hp right. Car has 82k miles, 5w50 oil, new spark plugs and air filter also. In the last 12 years it only did 10k miles (ims changed 11 years ago), coolant is clean, temperature sticks at 90 Celsius, oil pressure on hot engine at 1.1bar. It's a 1999 996.1 3.4L C4 manual. Very well maintained car.
I did today about 80 miles. In the early morning I cleaned the MAF sensor without noticing any difference, the problem was still there... applying full throttle from idle chokes and hesitates to raise the rpm smoothly.
Later this afternoon I disconnected the MAF and the engine ran just a bit better with the PSM and ABS mil on...so, I got home and connected my iCarsoft POR V2.0, deleted the DTCs and then the engine started to run worse than before. I did a log with my diagnostic tool and also took some pics and vid (click on the link to be redirected to YouTube).
I don't know if could be a faulty water sensor, Egas miscalibration, clogged injectors/filter, faulty fuel pump, AOS failure, or the cam deviation over 1k degrees (which is strange cause when I bought the car it was on -8.62deg fixed). No smoke wether on cold or hot, oil level just before full, ron95 gasoline, the car has been used and driven very softly within the last 12 years. I'll take out the oil filter next week to analyze it and looking for plastic and/or metal traces.Link to YT video of the issue
Anyone that had the same issue could tell me how did you repair it? Please don't tell me I need a new engine...I now that is a solution but can't spend over 12k right now on that lol
It could be many things. I did not look at the vid yet. Here are starter questions:
-what were the codes that you cleared?
-what does ‘well maintained’ mean? (Oil change every 6 months?)(fuel stabilizer used for garage queen?)
-how long has the engine been like this?
It could be many things. I did not look at the vid yet. Here are starter questions:
-what were the codes that you cleared?
-what does ‘well maintained’ mean? (Oil change every 6 months?)(fuel stabilizer used for garage queen?)
-how long has the engine been like this?
Let's see... It has been a garage queen, to give you an example...last oil change was last august and did only 80miles untill last Friday since then right when I bought it. Last owner had it for 12 years and did only 10k miles (most of them at the beginning and oil changes every year or 1.5 years). Well maintained regarding the car itself (interior, tires, brakes, oil changes...)
Untill 2008 maintenance was at the dealer (not sure if only at the dealer or somewhere else besides the dealer) but oil changes were on time by dates but we'll after 8k miles.
The codes i cleared were caused by disconnecting the MAF sensor only. Besides that the engine ran rough but there was no DTC and no MIL.
I must take out the oil filter and look up for other things than oil in it. Bought another filter yesterday.
Camshaft deviation is over 1k degrees which is weird to me. I road tested the car for 1st time last Wednesday and felt a power/torque drop after 5k rpm. Thought about the variocam pads maybe or other faulty sensor (no DTC either and cam deviation at -8.xx deg after a 30min drive)
Check the fuel pump Venturi tubes (in the gas tank). My engine had a similar problem when it was re-built by FSI about 4/5 years ago. It drove them crazy because they could figure out why it would only go lean at higher rpm’s. It turned out that the Venturi tubes had hairline cracks in the plastic…good luck
Check the fuel pump Venturi tubes (in the gas tank). My engine had a similar problem when it was re-built by FSI about 4/5 years ago. It drove them crazy because they could figure out why it would only go lean at higher rpm’s. It turned out that the Venturi tubes had hairline cracks in the plastic…good luck
Despite the excessive fuel consumption you think it could be those Venturi tubes? I did 80miles with 1/4 of a tank on a steady 65mph speed
After looking at your video sounds like you’re over fueling at idle/low RPMs.
Regardless, you need to get rid of that old oil,
drop the oil sump , take a picture of what might be in the sump and check back.
Rent a fuel pressure guage from autizone to verify fuel pressure, check air filter. That eliminated 2/3s of what it could be. Next would be buy a duremetric and learn how to read the telemetry. Or u can just throw money at it and waste time with internet arm chair suggestions.
Rent a fuel pressure guage from autizone to verify fuel pressure, check air filter. That eliminated 2/3s of what it could be. Next would be buy a duremetric and learn how to read the telemetry. Or u can just throw money at it and waste time with internet arm chair suggestions.
It's ok. I just got rid of the car giving it back to the seller. I just can't live with a ticking bomb behind me. Thank you all for your help
Check the fuel pump Venturi tubes (in the gas tank). My engine had a similar problem when it was re-built by FSI about 4/5 years ago. It drove them crazy because they could figure out why it would only go lean at higher rpm’s. It turned out that the Venturi tubes had hairline cracks in the plastic…good luck
Do both C2 and C4 have venturi tubes?
Did it give FRA lean numbers?
The AWD cars have the venturi tubes in the gas tank, the RWD cars do not.
The venturi tubes (there are two of them) siphon off some of the pressure/flow from the pump to suck gas out of two lower areas of the gas tank (one each side of the front driveshaft). They are sized so as to not starve the engine for fuel of course. If the corrugated hoses crack more fuel is "lost" to the venturi tubes than was scheduled so the engine can go lean at high power. The fuel doesn't leak out of course because it's leaking inside the fuel tank; it just doesn't make it to the engine.
The other problem that happens on the AWD cars is that a short U shaped line on the fuel pump outlet can split. This also reduces fuel flow and can make the engine run lean at high power settings for a small leak or make the engine not run at all for a large leak.
The venturi tubes are also called "Motive Flow Pumps" and are widely used in automotive and aerospace applications because they have no moving parts.
The AWD cars have the venturi tubes in the gas tank, the RWD cars do not.
The venturi tubes (there are two of them) siphon off some of the pressure/flow from the pump to suck gas out of two lower areas of the gas tank (one each side of the front driveshaft). They are sized so as to not starve the engine for fuel of course. If the corrugated hoses crack more fuel is "lost" to the venturi tubes than was scheduled so the engine can go lean at high power. The fuel doesn't leak out of course because it's leaking inside the fuel tank; it just doesn't make it to the engine.
The other problem that happens on the AWD cars is that a short U shaped line on the fuel pump outlet can split. This also reduces fuel flow and can make the engine run lean at high power settings for a small leak or make the engine not run at all for a large leak.
The venturi tubes are also called "Motive Flow Pumps" and are widely used in automotive and aerospace applications because they have no moving parts.
Damn good explanation. That's the best one I've seen. I imagine problems with either one manifest as decreased fuel pressure or fuel volume.
Does the GT3, which shares it's chassis with the all-wheel drive variants, have the same gas tank configuration and Venturi tubes?
Last edited by Otto Bonn; May 20, 2023 at 02:47 AM.
Engine chokes under acceleration
