3000rpm rev at hot start

Ok I feel that my baby is dying!

98 jetta 225,000 miles way overdue on timing belt

One day I got in the car and it revved up when I started it (car was warm), I thought nothing of it but immediatly noticed she was down on power.

It like throttle from 25% to 100% makes no more power, it's really a dog now.

If the car is fully up to temp. and I shut it off for 5 min., when i restart it it revs up to like 3000rpm and blows a big cloud of black smoke.

She desperately needs a TB and I am getting ready to do it, i have all of the parts.

My concern is that I also now have other issues like a faulty injector or the IP is going bad.

Has anyone heard of a 98 TDI doing this rev at start?

could it be that an injector is like hanging open and filling a cylinder with fuel when its hot, making it rev when i first start it?

no thoughts??

reving to 3000

Sounds like you are sucking oil into intake! Beware of a runaway! Check to see if turbo is leaking oil. Also clean the egr valve & remove intake & clean it while you are doing timing belt. Check your oil level.

sound like a good tip. sound like we need to check the intercooler as well

bump.........

check the coolant temp reading with vag-com/vcds.. if the computer thinks the engine is VERY cold it'll flare up like that when it's hot and have no power..

Good idea to check the turbo as well.. I would suggest pulling off the hose that goes from the upper intercooler pipe to the intake before you start it and look for a bunch of oil to come out when you fire it up..

Fuel shut-off selenoid!

Symptoms:
-Reduced power (Boost deviation code - Low)
-Revs up on start-up

You may only need a new selenoid o-ring, either way the selenoid needs to come out, replace the o-ring and this should fix your issue. If not you may have a bad valve/plunger, cost is around $113 for a new selenoid

O-Ring Part number: 068 130 139
Cost $1.92
http://www.worldimpex.com/parts/genuine-part-seal_44154.html

Fuel shut-off selenoid part number: 028 130 135 F
Cost $113
http://www.worldimpex.com/parts/genuine-part-fuel-cut-off-valve-tdi_177697.html


O-ring is under #3 in the diagram:




DB

Thanks for the input, sorry I was away from the scope for the weekend. I was suspecting something with the IP. I will look into the fuel shutoff solenoid and report back. I would like to fix this issue before tearing it down becasue it was running perfect. I will also be cleaning my intake and IC when I tear it down.

Update

First I have to thank Drivbiwire, not only for the explanation, but also the PN's and a fix!

I bought the new Cut-off solenoid and replaced it. The old one looked fine mechanically and the o-ring was not split.

As soon as I finished I started the car , it turned over for a few seconds as it primed. I checked for leaks and took her for a ride, immediately I noticed it was still down on power. So I drove it about 2-3 miles around the block and back to my garage.

I pulled out the generic scan tool I have (yeah I know Vag-com on the way), and checked for codes. None. I also tried to restart the car to see if it did the 3000 rpm rev at start, and it seemed like it started to but caught itself. I turned it off and restarted it about 3 times and hooked up my scan tool reading real time data (painfully slow sampling).

Before I even got out of the driveway I could tell the power was back!!!

So... I found it interesting that it did not immediately fix it until a few restarts. Is there some sort of calibration that occurs after a few start cycles?

Seems like it is just and electromagnetic solenoid, I didn't think it had any kind of feedback control.

I am very confused on how a sticky shut-off solenoid can cause the engine to over rev. All the solenoid does is stop the flow of fuel from the case into the fill port of the pump plunger. If it sticks closed it doesn't start and if it sticks open, it doesn't shut off the fuel. So what happens when it sticks part way that causes the rev at start-up?

My first thought is that if the fuel shutoff stays open, a little extra fuel can be injected during shutdown. It must be just the tiniest bit as the engine loses speed and can't inject in the ignition window.

Since there is a fuel shut-off valve it is reasonable to conjecture the control collar does not move (fast enough, or all the way to zero) during shut-down, relying on the cut-off to stop the flow.

You beat me to it ^^ essentially it leaks some fuel into the cylinders that does not get burned until restart.

In fact if you let it sit long enough it wouldn't do it. The fuel eventually vaporizes

That part I understood but... I assumed that the loss of power was due to it not completely opening.

This also confuses me, why then did it not work on the first test drive if it was fully open??

Maybe a fluke?

Also I just remembered my scan tool found "enhanced code" p1144, has anyone seen this (I will search it)

Do a search

just kidding - MAF open/short to ground.

Here's a thread for your enjoyment.

That's a reasonable explaination and I'll run with that for now.

Low power is due to the plunger on the selenoid "Throttling" the fuel entering the high pressure plunger. Enough gets drawn in for the car to run, but at full power you get a limited amount and a reduction in power. The loss of power is often just like what you get with a MAF failure and it will even trip a boost deviation LOW code. The code is the result of insufficient gas energy to drive the turbo to provide sufficient boost pressure, so the ECU cuts the fueling to protect the turbo as if there is a boost system leak! Now you have a DOUBLE reduction in power (MAF code/ limp mode + Reduced fuel entering the plunger = Horrible power)

The rev up is a simple explanation but the rev up is due to the Idle control circuit. Idle control handles from initiation of the starter and until the engine is at idle rpms. The idle control has authority to exceed the normal fuel mapping in order to get the engine to start and maintain smooth idle at 903 (or as specified in the coding). When you lower the IQ value too low ie less than 2.8 it conflicts with the idle control circuit and you get stumble when decelerating due to the confilict the low IQ creates between the two programs.

Anyway,

The rev up is due to the sticking plunger, the ECU watches the crank acceleration at 90 degree intervals on the AHU/1Z by way of a crank position sensor and a hall sensor mounted near the #3 crank journal. Keep in mind that for every rotation you get 4 samplings, with every 4 combustion strokes you read 8 samplings.

You had a jammed or sticking fuel shut-off selenoid hence...

If the Idle control does not see sufficient acceleration between samples, it increases fueling...then when the selenoid opens due to vibration or whatever was going on the engine suddenly fires the IQ value having been increased over the last 32-48 samplings is quite a bit higher than normal and by the time the ECU reduces the fuel control collar to resume normal idle control the engine revs up.

If you took this to a dealer they would have beat their heads against the wall! More than likely they would have diagnosed it as a bad injection pump...And there is a reason they sell that selenoid by itself, they do wear out!

Congrats!
DB

Jcrews explaination was simpler and according to Occams Razor, that one should be right.

The only thing that seems to be in conflict though is the early ALH warm starting map. It shows that fueling won't happen until a certain RPMs, but you are saying that if the ECU doesn't see the expected RPM increase it will increase fueling. The only way I can see a resolution to this conflict is that since the ECU isn't requesting fueling on the early ALH it isn't expecting an RPM increase so it won't increase fueling. Does that sound correct?

It's not that it isn't requesting fuel, it's that the startup condition map is a little off. It only affects the first few seconds of operation until the idle/run loop takes over.

I've watched the value on the bugged ECM, and it just starts with a low IQ, but after several seconds of non-start, it gives up and the IQ jumps to a high enough value to achieve start.

I started messing around with adaptation channel 05 and there may be a bit boundary in it, but I haven't sorted out exactly which values are where and what's being changed. The good news is that I was able to get hot starts down to near immediate because the startup conditions now start at or near the correct starting injection quantity. Cold starts are a little better too.

With the standard adaptation, the hot startup conditions would begin at or near 0 mg/h, no start, increase slightly, loop a few times, give up, jump to nearly 30 mg/H, then Grrrrk! clackclackclack...


drivbiwire makes sense too, I was going at it from the opposite direction.

Thanks again

Yeah It was a maf code, and it's because I unplugged it and drove it thinking it might be the maf but of course it was not.

I found it interesting that it did not set off a check engine light during or after but I did find it in the "enhanced code" section of my scanner.

The other problem the car has had since I bought it, is a missfire or rough operation between like 1300-1600 RPM. I had searched this before and didn't find much other than the fact that it was somewhat common on these cars. Probably just didn't use the right search terms.

Any thoughts?

I just dealt with this same issue on my 98 jetta the no power and rev on start up sometimes... Replaced the fuel solenoid and on the first drive it was still low on power. I then shut the car down and fired it up and off a few times, went out and drove it and its all better. Thanks for the info here i'm glad I found it.