The familiar 1500 rpm shudder ...

[GoFaster]

Lots of us have experienced that shuddering at part load and 1500 rpm, it seems to be a TDI "feature", even with stock cars. With my recent chip upgrade, the shudder became a lot worse, enough to prompt an investigation to find the cause. And I think I've found it, and I have a work-around to cover it up.

The problem is a combination of the simplistic design of the EGR system, and very close approach of the EGR map and the "smoke" map at that particular speed and load.

Background on how the ECU figures out the injection quantity:

The engine speed and the pedal position are used to calculate a preliminary injection quantity. Based on this preliminary quantity, the ECU consults the EGR map to determine a desired amount of intake air. If the actual amount of air entering the engine is less than this desired amount, it closes the EGR solenoid valve. If the actual amount is more than this desired amount, it opens the EGR solenoid valve. (More later on what happens after the ECU does this.)

After some more manipulation, the ECU consults the "smoke map" to determine the maximum permissible fuel quantity based on measured intake air. If the preliminary quantity calculated above is less than this maximum, fine. If the preliminary quantity is greater than the maximum permissible, the actual injection amount is limited to the maximum permissible (i.e. it cuts the fuel back).

Inherent in this design, is the requirement that the maximum permissible amount according to the smoke map, should always be greater than the "desired" amount according to the EGR map by some margin, for all speed and load conditions. Otherwise the result will be driveability problems (more later).

Now, more about that simplistic EGR system.

The EGR solenoid valve is an on/off device. Solenoid "off" ... atmospheric pressure is applied to the EGR diaphragm. Solenoid "on" ... vacuum is applied to the diaphragm. What this means, is that the EGR valve doesn't find a fixed setpoint and stay there under a given speed and load, it constantly hunts back and forth around the setpoint. The EGR valve cannot be stationary (unless it is fully open or closed, of course), it can only be opening or closing.

And that's why the EGR map and the "smoke" map must not approach each other too closely. If they do, the fluctuations in intake air volume caused by the EGR valve opening and closing will cause the "smoke" map to start limiting the fuel quantity up and down as the measured intake air fluctuates. Presto ... you now have a driveability problem, a "stumble".

So, what to do about it?

The cheap home-grown fix is to do something like what I did, and install a restrictor in the vacuum line to the EGR valve to dampen its movement. I hunted through my parts bins and found a suitable small orifice ... a pilot jet for a Yamaha FZR400 carburetor, Yamaha part number 3H1-14142-15-00. One of them didn't quite do the trick, two of them (one at each end of the line to the EGR valve) seem to have eliminated the stumble. I'll find out tomorrow for sure.

The downside, and this is how I know the smoke map is limiting things, is that if you nail the diesel pedal in the speed range where you'd normally have the stumble, you now get a noticeable delay before anything happens. Reason ... the EGR valve is being commanded to close, but it's taking a while to do it, because of the extra restriction in the line. It's no bother when shifting gears, because the EGR valve also takes a long time to open. I'm going to drive it like this for a day or two, I might end up with a compromise that has a little bit of stumble but not as much hesitation.

Obviously, the use of the Epsilonian device in its present form, is out of the question in this application. It can presently only give an EGR "on" or "off" response, and if the smoke map limits the fuel every time the EGR is "on", the result is big-time stumbling.

So what's the real solution in place of my cheap fix?

Since a servo-controlled EGR valve is out of the question (some GM cars have this), it's probably for the chips to be remapped slightly, to increase the intake air volume at light loads to get the EGR map away from the "smoke" map. This is not something for the home mechanic ... and if I get satisfactory results with my little orifice in the line, I'm not going to worry about it.

Brian P.
'96 Passat TDI mit UPsolute

 

[valois]

A simple capacitor will rectify the epsilonian device by smoothing out the sine wave being produced in some cars. I will do some testing tomorrow and report back what I have come up with.

 

[GoFaster]

With the restrictors in the line, the shudder is reduced but not eliminated. There is still a very narrow range of engine load where bucking occurs. I definitely need to reduce the amount of restriction from what I have now, there is too much lag.

I'm starting to think that the EGR valve itself is sticking, rather than opening and closing smoothly. Time to clean the intake manifold and EGR valve ...

Brian P.
'96 Passat TDI mit UPsolute

 

[diesel des]

My tdi bucks with the EGR platted off. It doesent do it with a shot of Millers fuel treatment in the tank though. Very strange.

 

[GoFaster]

The thing about theories is that some of them are right, some of them are half right, and some of them are so full of crap that it's overflowing. Looks like this one is in the latter category

Today, for the trip home, I said the heck with the check engine light, and removed the EGR from the equation by pulling the hose off the EGR valve. Driveability went right back to square one, same as with the EGR Buster in place. In other words, the stumble ISN'T due to airflow fluctuations. The engine wants the EGR in place. Interestingly, it does run better with the EGR stabilized with a single restrictor.

So what else gets affected by EGR? Intake temperature, certainly a mixture of blended exhaust gas and outside air is a higher temperature than outside air only. (It's about -5 C outside right now.) And what does that affect? Ignition delay...

Time to find some cetane booster to feed to this hot-rod TDI.

Interesting side note: despite the disconnected EGR hose, the check engine light did NOT come on during the whole 50 km trip home!

Brian P.
'96 Passat TDI mit UPsolute

 

[BG]

I use to have the same shutter at low RPM's
Mine completely went away when I got rid of my stock flywheel set up( 2 piece). Put G60 flywheel set upv ( 1 piece) with VR6 clutch. Now smoth as silk from 1200rpm's up.

 

[Gary Miyakawa]

In playing with the TB, I found that I can induce or prevent the shudder by adjusting the "S" pot (I believe that is injection timing)... With it set to where I "think" it should be (based on Valios adjustment suggestions) I do get shudder as the temperatures drop... I've probably had to back off the "S" pot about 1/2 turn with the recent temp drop from 75 to 45 degrees...

Gary

 

[SkyPup]

With EGR the ignition delay is less due to the heated intake air, without the EGR ignition delay is more due to the cooler air.

You guys without the EGR should be even more appreciative of some good cetane boost to cancel out the ignition delay.

 

[BoraTdi]

So, what have we determined here? Is it the clutch, or the EGR?? I just got the Upsolute upgrade this week, and my shudder started on day #2. I did stomp the pedal in 5th a few times right after I got the chip, with no issues. But on the second day it started happening. This tells me it's the clutch, as I would suspect any fuel or timing issues to show up immediately. Time for a G60/VR6 setup I guess.

Will
99.5 Jetta GLS
(BTW Rene, it does smoke a little more under full load, but that makes it all the better to make tailgaters back off

)

 

[GoFaster]

I'm starting to think that there are two separate things going on here: one is the shudder at 1500 rpm, and the other is a misfire under light-load conditions regardless of engine speed. I have had it happen as high as 120 km/h / 2600 rpm. This misfire seems to have something to do with being Upsoluted but not everybody mentions the problem, so there's more to it than that.

At moderate to full load, my car has no issues, it runs fine, and this would suggest that the clutch friction plates have nothing to do with it.

BG brings up a good point. Clutches generally have some damper springs inside them to absorb the uneven part of the crankshaft rotation and prevent it from being transmitted to the drivetrain. Any spring/mass system will have a natural oscillation frequency, and perhaps this one is at 1500 rpm...

Brian P.
'96 Passat TDI mit UPsolute

 

[scott matre]

Did some playing around with my 98 upsoluted TDI the past few days and noticed that the 1500 rpm stumble only happens in 5th gear.. Does this make sense?? I after 1st noticing it, I tried like hell to replicate the stumble in 4th and 3rd with no success.. Can some of you guys check to see if you can get it to happen in 3rd or 4th...
At least we'll know if I'm nuts, or we get a little closer to the real cause.

 

[BoraTdi]

Mine only happens in 5th as well, and mine is an A4. My 2 cents.

Will
99.5 Jetta GLS

 

[Karl Roenick]

I remember at least one guy, I think he was from Europe, who tried disconnecting egr on his older Passat and it wouldn't run right. Could this particular hesitation be caused by the old turbo model? Do A3 Jettas and Golfs have this problem with EGR disconnected? Or maybe it's Passat-specific and related to that TSB that involved changing injectors and the ECU?

 

[GoFaster]

With the stock chips, my car ran just fine with the EGR disconnected. (Other than the 1500 rpm shudder, which has been there all the time no matter what else is changed. Upsolute or not, EGR connected or not.)

I suspect that the shudder is only apparent in top gear because in the lower gears at that load condition, the car will accelerate through the hiccup so quickly that it's not noticeable.

It certainly isn't Passat-specific. Scott, a couple of posts up, has a '98 which therefore is not a Passat. BoraTDI mentions it for his A4, so it isn't turbo-related either. It's just odd that a lot of cars do it, but apparently not all of them...

Brian P.
'96 Passat TDI mit UPsolute

 

[dzcad90]

I think BG hit it right on the nose here. Could it be that the engine is putting out way more torque than it normally would, and that's where the shudder is coming in?Upgrade the clutch and your problems will likely go away.

Yesterday I posted that removing the EGR buster cleared up the shudder, but after taking a long trip yesterday, it was still there, just not as bad.

Also, after hooking up the EGR again, about 30 miles into my trip the CEL came on. It then went off after I stopped and started the car twice at the mall. It hasn't came on since. A firend has an OBDII tool, so I'm gonna see what the history code is later today hopefully.


------------------
1997 Jetta TDI
Upsolute Chip
K&N Filter
Descreened
Broken Timing Belt @ 72,800 (15,100 miles on belt)
New Cylinder Head

caddy@udnet.net

 

[Turbo231]

My Beetle is stock, and it has the shudder, with or without the buster. I don't think it's a buster problem. I feel that it is a fuel problem.If I add a good dose os cetane booster it stops. I've been thinking about it and mine shudders as I'm slowing or at a load under 2K. I reminds me of race cars I've tuned that had what I call a lean buck.
My cobra would just about make you bang your head on the steering wheel. Could it be that VW was trying to keep the smoke to a minimum and has the smoke map extended down too low?
Could someone try a chip with more low RPM fueling? I know a diesel is different but it acts like problems I've seen before in gas burners.

 

[VeeDubTDI]

It shouldn't be "lean buck" on deceleration becuase there is no injection of fuel whatsoever when the car is slowing down in gear. Under 0 load conditions, it should be as smooth as it's going to get.

Here's a weird little scenario for you all to evaluate. I have a little 1500 rpm hiccup... in 1st gear. That's the only gear that really does it. I'm thinking that it's quick adjustment of the VNT system on the turbo that is causing rapid changes in acceleration. I have not noticed anything in 5th gear. Weird.

------------------
2000 Yellow New Beetle 1.9 TDI 5-speed

 

[BoraTdi]

Update: my A4 is now doing it in both 4th and 5th gear. It was spewing black smoke today too, but i suspect that may be another issue. First, 2nd, and 3rd are still fun as hell though

Will
99.5 Jetta GLS

 

[DZLKopf]

NEWSFLASH

Gas Stations Switch to Winter Formula Diesel

Diesel Owners Become Paranoid Thinking Performance Probs are Related to Recent Mods

Film at Eleven

 

[GoFaster]

Not in this case. The Upsolute chips went into my car with the fuel gauge at the halfway mark. That first half tank of fuel done with the stock chips - No misfiring. (And the car has never misfired with the stock chips, all last winter.) The second half tank, with the Upsolute chips installed and no cetane boost, was very very rough.

The car runs better with cetane boost, but still misfires occasionally at highway speed. I'm quite positive that if I put the stock chips back in, it will run fine (albeit more slowly!). Let's play with that VAG-COM at the GTG this Saturday. Betcha there's too much injection advance under highway cruising conditions.

Brian P.
'96 Passat TDI mit UPsolute

 

[Lance Allison]

I also have the occasional shudder, except mine occurs only in 5th gear at bewteen 44 and 46 mph under light acceleration. The shudder is not heavy, but you can feel it.

Don't experience it too much, as 95% of the time, I'm in 5th gear at a much higher RPM. I can adjust the TB so that the shudder almost disappears, but personally, I'd rather have the extra power.

I drove DZCAD90's car and the shudder is definitely more pronounced, at a higher RPM, and occurs under steady or light throttle. In fact, if the cruise was set at 58-60 mph, the car would shudder pretty bad. Give it a good stab at the throttle and it stopped.

Think we'll give his car a healthy dose of Amsoil Cetane Boost and see what happens.

------------------
Lance Allison
LDA4793@UDNET.NET
98 Jetta TDI
150,000+ miles
Upsolute Chip & Tuning Box
Epsilonian Confederate
K&N / Descreened / Desnorkeled

[This message has been edited by Lance Allison (edited November 27, 2000).]

[This message has been edited by Lance Allison (edited November 27, 2000).]

 

[valois]

Check your injection timing fellows. DZLKopf is right in this regard, you did not hear much about this in the summer, the fuel has changed, even in NC, I am getting less MPG and worse got some fuel with water in it last night, I was on a 200 mile trip and did not know the station so I only took on 3 gallons of fuel, did not have my additives with me, talk about crap. I am sure it was water logged as we have had heavy rains here this weekend, I developed major stumble, I drove 50 miles till I came across an Amoco I am familiar with, filled the car up, and presto, problem solved. It smoothed right out. Cetane boost is neccessary when your timing is close to the edge using crappy NA fuel, time for skypup to say"I have been telling you so, for the last 3 years.

" I believe if your static timong on the pump is too high it may need to be backed down. Let me know what you find out with the vag-com Go Faster. I have to do a filter change today, and run some diesel purge to clean my pump and injectors out. Also do not rule out the MAF sensor on this one. I do not believe it's a problem with the program, too many are running well. The upgrade can and will acentuate an existing problem as I saw with Red's car.

 

[Turbo Steve]

Gofaster and Valois' last posts make sense, in that I too believe there are 2 or more different prblems here.

Some areas to consider might be:

1. In another thread regarding hesitation symptoms, Valois said: "The sysptoms are a CEL light, triggered when over 10 psi of boost is demanded in 5th, the ECU reports that the MAP is outside of expected limits."

2. Water in the fuel is an obvious one.

3. Also, don't forget about leaves blocking the intake tract's snow screen, which will cause hesitation problems at high speeds / RPM's.

4. Cold temps can create havoc on a TDI who's injection timing is off or who's Tuning Box was set during hot temps.

-steve

Perhaps we should start a thread called Hesitation 101!

 

[GoFaster]

It's quite possible that the static timing on my car is within an acceptable range with the stock chips and the extra timing advance throws it beyond the acceptable range. This would certainly explain why some cars do it and some cars don't. We've certainly seen our share of out-of-spec cars even from brand new...

I certainly hope that's the case, because then it's something I can fix. Will keep this page posted as always.

Brian

 

[BoraTdi]

I added about 4oz Amsoil Cetane Boost to my fill-up on Saturday, and the bucking is still there. I will remind once again that I never had any stumbling whatsoever before the chipping, and I have driven my car pretty hard for 37,000 miles. If it IS the injection pump timing, which I am inclined to believe, what's the quickest how-to for adjusting it? I have my T-belt coming up in 3k, so maybe I should have my service guys look at this and adjust it?? They aren't the usual Stealership btw, they are good and I have known the guys for 9 years.

Will
99 Jetta GLS

 

[VeeDubTDI]

You might be able to talk them into retarding it a degree or two. That should compensate for the chip advancing it too far. It's worth a try anyway.

------------------
2000 Yellow New Beetle 1.9 TDI 5-speed

 

[RED]

Correct me if I'm wrong valois, but I think my 98 jetta has timing on the border of being too retarded. This based on valois hooking up his VAG-com. I also have this bucking in 5th at around 1500 rpm. It is strongest at 1500 and smooths out at around 1900 rpm. It also happens in 4th, but not as pronounced.

 

[valois]

Red if I recall we tested yours and found it to be on the very lower limit of the bently chart. I do not think anyone has coorelated these numbers to actual timing degrees, drivbiwire posted something on this previously, he has also indicated it maybe a cam timing issue, I believe you had an independant garage change your belt, I believe Drivbiwire is correct, and it's possible both your cam and pump timing are out of spec. We are having a timing belt replacement party here in Charlotte this weekend, I have the tools to adjust the cam and lock it in as well as the pump locking pin, can you make it over here? We'll check yours out and see where it's at.

 

[GoFaster]

There is a current thread on adjusting the timing on the Maintenance forum, because I'm reasonably sure that I have to adjust mine, too.

 

[Craig]

Over on another thread I mentioned that so far my ride is performing smooth as a baby's bottom (http://forums.tdiclub.com/NonCGI/Forum4/HTML/001941.html). What I forgot to mention is when Upsoluted, I had DZLKopf check my timing when I made a test run. It is near the outside edge (advanced) of the map in Bentley.