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Next, you might hook up a fuel pressure gauge and run the testing procedures, to rule that out.

Beyond, that, i would test your diverter valve; the can stick (from heat warping; possibly worse when at operating temps), not hold vacuum and the N75 valve, which controls the boost, also seem to malfunction when hot. Some, have tested the N75 with a multimeter and artificially introduced heat, to the part from a heat gun or hair dryer and then, noted the changes in the reading; as a attempt, to replicate, real world operating conditions.


I had a boost hesitation issue, which i had a hard time figuring out; after testing, it turns out, my dv was sticking and my n75 was not performing as it should, the new replacement parts fixed the hesitation issues.

My boost control was restored and then, i had full, smooth boost, once again.


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N75: testing:



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Discussion Starter · #24 · (Edited)
So..I reinstalled the factory MAF and it seemed fine and ran really hard for a day, then a really hard cold start set a bunch of codes including an immobilizer code and random and specific cylinder misfires . I reset them as I expected them to come back if the problem persisted. Yesterday though it ran like garbage like never before. The car is virtually undriveable on the highway. Next week Ill have the funds to replace the MAF and fuel relay with a recommended brand and probably a diverter valve as well as it seems If I roll the throttle on from a midpoint (not 0 to wot) it won't hold boost, but will hold more boost with a wot snap. It also drops boost accelerating then holding the throttle at some mid point for cruise. Essentially running on a hard vaccuum where it takes more throttle to cruise than normal by far. Ill probably order whatever other sensors I need. A new n75 as well. Maybe coolant sensor. Anything else I should consider? Ive gone through all my hoses, had them apart and replaced anything shady and replaced clamps after cleaning the air charge piping connections.edit-went looking for the 034Motorsports DValve. Not being familiar with vdub equipment I realized I already had one! Lol. So I pulled it apart and cleaned it up and reassembled it. Ill order a seal kit for it, but it was pristine inside.
 

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Please, post any codes; you get and not just the descriptions.

For the reinstalled stock vw Bosch maf; i would keep an eye on the readings and cleaning it using some crc maf cleaner or 90% isopropyl alcohol, couldn’t hurts, before replacing it. If it is reading in spec; theoretically it is working ok. Sometimes, we have seen a bit debris will get stuck inside the sensor, causing reading problems and cleaning, can flush it out.


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For the dv a simple check for holding vacuum and using a vacuum tester or if not available, push in the diaphragm and holding the port, confirm it stays open, until released.


In my case, m dv held vacuum but when the vacuum was released, i could feel the piston, binding as it opened, slowing down its actuation. Comparing it to a new oem Kayser A710N; the new one opened smoother and no binding. Reading online, many say the heat warping the plastic and can cause the binding issue.

Since then, i have upgraded to a billet aluminum Forge DV 008 art# FMDV008. I got it used, installed a new revised piston with the bottom having a rubber seal and a rebuild kit.


This was a improvement over my replacement Kayser 710N, which performance; seemed, to be degrading over time and possibly, getting warped like my original did.


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The n75 is easy to test and as noted; you might use a heat gun or hair drier; to add heat to compare resistance difference.


Note: on my N75, cold my original n75 the resistance seemed to be in spec and not much different then a new one. I didn’t add heat; i read about that testing addition until later but it makes sense to do so.

The replacement made a difference; as noted, a new dv and n75, eliminated a boost hesitation issue i struggled with for years. Off the line, the boost would dip, engine seemingly “fall on its face,” then recover; post new parts install, the boost was improved, more responsive and smooth. It was very satisfying fixing this problematic issue; very frustrating, driving around a 1.8T with boost issues, no trouble codes, never knowing what the problem was and my initial testing of the parts, did not indicate total failure, following the VW service manual testing procedures. I’ve come to the conclusion, that many parts can be in a “soft failure mode”; meaning, functional but not at their optimal levels, which can adversely affect normal boost operation.

This, was two important things, that really helped my 1.8T run better, related to boost but there were and are many other things to go through, check as well. All the various parts, of the engine control system, fuel, induction, ignition, etc; all play a role in a good, solidly performing 1.8T.


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Discussion Starter · #28 · (Edited)
OK here is an update. During the shut off event:
Watched voltage at the pump that I replaced to get rid of this issue. Seemed OK during event. Did not set min and max on my meter, so at a glance I saw 13+ so it meant the relay was behaving at least. figured fine.
Watched power and ground on the coils. Stayed the same during the event.
Watched injector pw both on my meter and ibdelelven. Seemed normal to high during event (trying to add fuel though).
Watched duty cycle of ignitor during...Seemed normal.
.....Starting to make me crazy.
Watched B1s1 HO2S. It happened and I watched it ping to 4.3v (LEAN!)
Back to the voltmeter on the fuel pump: finally noticed voltage went high during the event to almost 13.8volts...OOPS! Must have missed that last time. That meant the pump is going open and voltage stops dropping across the terminals of the pump. So I took it out, wiggled the wires a around. Ohmed them for continuity..Seemed OK.
Finally bit the bullet and found the parts I needed to hook up a fuel pressure gage. All this time I had been thinking if ignition is cutting and not fuel, it should pop in the exhaust when it comes back on, and it doesn't. All this time the only code was p1297 (charge pressure to throttle) and only set under boost.
So I finally hooked up my gauge with the face tied to the wiper blade and drove it for 2 days. Nothing Nothing then boom! Watched the needle go bonkers and then drop like a rock. Car dead then it Seemed to actually get worse. The dead time was increasing. Got it back to my house where it happened idling in the driveway this time I had the back seat out and could hear it stumble and stop and listened to all the fuel rush back to the tank. And then I swore a whole bunch as that pump was the first part I replaced to solve the problem. It was a Quantum Fuel Systems drop in (to factory housing) pump that I am looking for the receipt for now. It sure is pretty! Number was HFP A35HV 051218.
Before I found this forum I found this pump at the beginning of the saga. I guess I know now. So! I ended up putting an AEM generic 340 pump in from ECS Tuning and the car is like a different creature. All that effing around and it was the first part I replaced. It runs hard on the aem pump. I've used them with good luck on other cars. I'll try to post a picture of how I rigged it on the thread but it seems like my cell keeps choking on the process. I still have a bit of a driveability issue where if I whomp the throttle I get full boost but if I go from part throttle to wot it will not go to full..something 50-75% of full. I have an older n 034 diverter that I think needs a rebuild. I have to let off and pin the throttle then it gets full boost. but it isn't falling flat on its face any longer and otherwise runs MUCH harder than ever. It will be nice to not have to worry about it dying whenever it wants.
 

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Any testing on the N75? Solid testing, sometimes; you have ti hook up a gauge and drive around, until thing malfunction. For fuel pumps; aftermarket junk, are problematic; we have a ton of threads like yours and they always seem to cause drivability issues! We have found oem Bosch and VDO pumps; work the best. The aftermarket pumps, seem to not seem to be able to maintain needed volume under load or are are defective out of the box. Any reason, why you are choosing; to use aftermarket fuel pumps?
 

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Discussion Starter · #30 ·
I went with a pump I have had good luck with previously. I think that canister with the hot return fuel wrecks the oem pumps. There is always a pile of oem canisters on the bench at the local vw shop. I bypassed the n75 ports to a simple boost controller and left it plugged in. Put a weaker spring in the 034 diverter and the car has amazing power now and appropriate afrs up to redline (maybe a bit rich) up to 22g/s+ on the MAF at about 13.5psi boost. (It happens much faster now so it is hard to get good numbers without logging) Runs great. A little bit of a sharp drop of boost going from full to part throttle but it runs 200% better now and is so much fun to drive. I do think I have a small boost leak somewhere still. Front cooler and all new plumbing will be next. The ONLY code I get still is the p1297 charge pressure code sometimes..every 2 or 3 weeks I get a code for the cat efficiency if my temp wrapping to the high-flow cat gets wet. I have the exhaust wrapped from the manifold, turbo, downpipe right up to the cat flange and it seems to heat up very well. It's all 3inch tubing to a dynomax glass muffler. I'm tired of messing with weakly designed systems. I'll make it better or I'll make it dead. If I blow it up it will get a tesla drive like the 911 I did a few years ago. Especially with 93 going for almost $6/gal here. Yikes. Fun cars to drive and beetles are cute but very frustrating to work on.
 

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A MBC, adds a whole other level of potential troubleshooting issues; are you running a tune or other upgrades?

Anyway, it sounds like you are making progress; things are better, good work!


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Discussion Starter · #32 ·
Thank you for all the help. I think many of these oem parts are barely better than aftermarket cam and crank sensors and plastic bits going bad at 100k? Stock pumps don't seem to last either and I have run aem pumps on a bunch of cars with zero trouble for years. This $100 pump will serve me better than a $300+oem pump for sure. No more hot return fuel dumping on the pump body....
I'll get my afr gauge installed so I can take a better look but if it isn't working correctly I'll throw a megasquirt in it and be done with all the trouble. I'll make a jump box like I usually do so I can plug the factory ecu in for emissions.
Thanks again.
 

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I don’t have any experience with aem pumps; we do our best to recommend, oem brands and parts, that we have seen that works around here. Through many troubleshooting sessions; the aftermarket parts have failed, (like your fuel pump), caused drivability issues and the oem parts, have restored things to correct functionality.

When getting past 100k and 15-20+ years old; a wide range of parts, will need replaced and the 1.8T, needs to be gone through, to get running right.

Based, upon my own experience; it can can takes a series of troubleshooting sessions and refreshing multiple aspects of the engine (boost, pcv, ignition, fuel delivery, etc.) to sort things out.

These older mark iv platform vw’s; can keep you busy, a challenge to keep running on the road and they definitely, had their issues, even when new.


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Discussion Starter · #34 ·
I don’t have any experience with aem pumps; we do our best to recommend, oem brands and parts, that we have seen that works around here. Through many troubleshooting sessions; the aftermarket parts have failed, (like your fuel pump), caused drivability issues and the oem parts, have restored things to correct functionality.

When getting past 100k and 15-20+ years old; a wide range of parts, will need replaced and the 1.8T, needs to be gone through, to get running right.

Based, upon my own experience; it can can takes a series of troubleshooting sessions and refreshing multiple aspects of the engine (boost, pcv, ignition, fuel delivery, etc.) to sort things out.

These older mark iv platform vw’s; can keep you busy, a challenge to keep running on the road and they definitely, had their issues, even when new.


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Ha! No kidding. Just as I had success with the fuel pump with one day of trouble free and fun driving, it started idling funny. Watched the alternator and load data and it was dropping below 13 at idle with 100% duty cycle with virtually no load and under 12 with ac, lights on. Then the bat light started flickering. Found a bosch reman at Napa. Seems to have done the trick. Removed it through the headlight hole. Took a bit more than an hour. It took more time to adjust the alt mount spacer bushings than anything. I also replaced the lower throttle charge hose spring clamp with a tbolt clamp and may have solved a small boost leak.
Thanks again.
 

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Whew, another issue; marked off your list; charging problems, can cause all kinds of problems, that make the computers go goofy! Which, can make troubleshooting things confusing; glad you got it fixed.

PS: what scan tool; have you been using, to view things, like the “duty cycle”, etc?


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