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Go Back   TDIClub Forums > TDI Model Specific Discussions Areas > VW MKIV-A4 TDIs (VE and PD)

VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old August 5th, 1999, 04:46   #1
Russell Parr
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Join Date: Mar 1999
Location: Atlanta, GA USA
Fuel Economy: 55 mpg Hwy
Default Early morning engine bucking or lurching solved!

After having the dreaded stumble or bucking on the first run of the day for the Tdi, I noticed that running cetane boost cured the problem. I have run 3 tanks of cetane boost from Amsoil and have not had one instance of the engine bucking in the morning. Nothing else has changed except that variable. Evidently the bucking is similar to a gasoline engine misfiring or running a to "rich" mixture. Since the engine is cold as well as the fuel the thermal efficiency should be at its lowest. The low cetane in the USA must agrivate this condition and give us the dreaded bucking. I can only deduce that increased cetane helps the fuel to burn quicker in a low thermal state. While this may not be the total "scientific" explanation to why this seems to cure the problem, on a cause and effect relationship it is working and continues to work.

If anybody else is having this problem give it a try and see if it helps you.

Chow
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Old August 5th, 1999, 06:53   #2
Ric Woodruff
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Join Date: Feb 1999
Default Re: Early morning engine bucking or lurching solved!

Congradulations Russell! You found a reason to actually justify using a cetane booster.

Also, it is funny that you mention the problem today because just this morning, I experienced the very same thing. It has not happened for probably 6 months, and may not happen for another 6 months.

If the problem is really due to low cetane, one would think that a particular tank of fuel would be especially pesky until you fill up again. Do keep us informed. This is an especially interesting observation.

I had a 4 cylinder 1981 Ford Fairmont that had a "Feedback Fuel System" (computer controlled?) that behaved the same way. It did not matter if the engine was hot or cold. Being in an accelerate or decelerate mode, it ran well, but under steady state conditions, it tended to buck like the TDI occasionally does. This leads me to believe that under certain conditions the ECU goes into an unstable feedback mode.

I have never seen the problem when the engine was warm. Different engine parts heat up at different rates. I believe that the problem is related to that fact.
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Old August 5th, 1999, 09:09   #3
jaydhall
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Location: Aurora Colorado
Default Re: Early morning engine bucking or lurching solved!

Russell,

I have been thinking about the amsoil stuff. It seems pretty good. How much is the ctane booster? I am thinking of becoming a 'dealer' and getting a price break.

Mickey,
Now this is not the kind of dealer you spoke about in the other thread. So, don't be telling the Police to look out for a Siver Jetta TDI! (I do not fit the profile...)
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Old August 5th, 1999, 09:15   #4
Russell Parr
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Location: Atlanta, GA USA
Fuel Economy: 55 mpg Hwy
Default Re: Early morning engine bucking or lurching solved!

jaydhall,
I believe the cetane boost is about $4-$5 per can and will treat between 100-200 gallons of fuel. I use 2oz per tank full. If you will give me your address I will send you the form you need to sign up as a dealer and send to Amsoil. If you can email me direct or want to phone me thats fine.
(770) 435-7970 ext. 4567
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Old August 5th, 1999, 13:53   #5
Farmboy
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Location: Bloomington (Minneapolis), MN, USA
Default Re: Early morning engine bucking or lurching solved!

The previous thread on lurching kind of surprised me, as I hadn't 'noticed' it myself, but I filed it away in the back of my mind. After reading through another thread or two discussing glow plugs and pre-heat I stopped paying attention to my light and started the car right up and drove off as soon as I felt I could trust that the oil pressure was up. Even when the temp is in the 70F/24C range, with a cold car at light throttle I experience mild lurching.

It's nearly always there, but I notice it much less when I take the time to let the preheat run it's course and let ther car run for a minute or five before rolling. Early on in life with much less refined vehicles I had learned that certain throttle positions with a cold engine were to be avoided, depending on the situation.

A person could develop a response to floor it when the lurching starts... but that will help shorten your engine life. Instead I developed an automatic response to shift up through the gears letting the engine stay basically at idle until I achieved the speed I wanted for the short term or until the car was warm enough to drive normally.

Also, I learned to 'feather' the accelerator when necessary - a skill that has saved me when one of my el cheapo cars has wanted to die in front of approaching traffic.

I also learned that if I don't 'tripod' my right foot against the tunnel, feedback through my foot can make the lurching much, much worse.

Does any of this make sense?

Of course, as I am learning more about the potential of my car I am getting more interested in using Cetane booster anyhow, but the above is my 'Plugger' response to the problem for the near term.

------------------
98 Jetta TDI 5spd - 2200 miles/mo

[This message has been edited by Farmboy (edited August 05, 1999).]
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Old August 5th, 1999, 14:50   #6
Stinky-D
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Location: New London CT
Default Re: Early morning engine bucking or lurching solved!

Will Dr. Diesel boost the cetane rating enough? Or do i need to get a special 'cetane booster' additive?
thanks
d
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Old August 5th, 1999, 16:39   #7
Jimmy
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Default Re: Early morning engine bucking or lurching solved!

I hate to raise the Glow Plug issue, you may want to take a peek at some new info:
http://www.vw-industrieverkauf.de/en...echnik-eng.pdf

Page 26 mentions the "afterglow" phase. If it is correct, the glow plugs are turned on for a short while whenever the engine is started, and swithced off above 2500 rpms. One surmises "afterglowing" also ceases when a coolant temp comes up a bit.

Now the ECU controls the turning on an off of the Glow plugs, and can sense if one fails. If it were fooled that your temp was above a certain value (bad coolant temp sensor perhaps?), you may not be getting any "afterglow".

It may be useful to tap a LED into the glow plug power circuit after the relay. The spot to so do would be a heavy red & white wire, form either terminal 6 or 4 of the Glow plug relay (#180). I had a look at schematic of that area at a dealer as I planned to do the same some time.
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Old August 5th, 1999, 16:47   #8
mickey
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Default Re: Early morning engine bucking or lurching solved!

For that matter, just revving your car above 2500 rpms right after starting it will shut off the glow plugs.

I wish I understood all this "bucking and kicking" stuff. Where do you people live that you even need glow plugs in the middle of the summer, anyway? Antarctica? I've never waited for the glow plug cycle if the overnight temperature stayed above 40 degrees F. or so, and I've never experienced any problems. Certainly the car doesn't run quite as smooth when it's cold as it does after warming up, but it's a diesel engine not an electric motor. If it's a nice, warm summer morning and your car is actually "bucking and kicking" and not just running slightly rough then you have a mechanical or electronic problem of some sort.

-mickey
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Old August 5th, 1999, 17:55   #9
Paul
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Join Date: Mar 1999
Location: LaGrangeville, NY, USA
Default Re: Early morning engine bucking or lurching solved!

98 Jetta TDI, ~26K miles, a little over a year old -

I never had any "bucking" or engine stumble of any kind, through summer or winter, ever !
In fact, compared to my 91 Jetta IDI, the smoothness at startup was really amazing to me. I always get in, hit the key, wait for the light to go out, start and drive away immediately. No cetane boost, stock everything,and I fill at a Citgo that has a diesel pump that says "40 cetane minimum" (yuck !!) Could be something wrong with your car ?
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Old August 5th, 1999, 18:55   #10
Lug_Nut
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Location: Sterling, Massachusetts. USA
Fuel Economy: Which VW diesel? There have been Twelve of them. Now 13!
Default Re: Early morning engine bucking or lurching solved!

The glow plugs remain on even after the dash indicator goes out. They will go off at engine speeds above 2500 rpm, but come back on when the speed drops below that point. They will be in this mode until the coolant is above 140 degrees F, or as near to that point as I can guess from the temperature gauge. I wait for my glow plug indicator to go off before starting the engine, it just gives me time to ensure that the other idiot lights are operational. Still living with the buck.

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Jonathan Bartlett
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Old August 6th, 1999, 05:49   #11
Warsaw Falcon
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Default Re: Early morning engine bucking or lurching solved!

I knew there had to be other buckers and kickers out there! I have been living with this annoyance for 8 months; the dealer claims it's "normal for diesels", to which I reply HOGWASH. This is my 5th VW diesel, and the first ever to have this problem. I'll try the cetane booster, as well as monitor the glowplug cycling. Keep the input coming! I wonder how many of us would put up with this problem if we were driving a Neon!
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Old August 6th, 1999, 06:16   #12
SkyPup
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Default Re: Early morning engine bucking or lurching solved!

DI DIESEL GLOW PLUG OPERATION:
ALL TDI ENGINES INCLUDING 96-97 PASSAT, 97-98 A3 TDI JETTAS, 98-99
A4 NB TDI, 99 A4 JETTA TDI, 99 A4 GOLF TDI
VW ENGINE CODES - AHU, ALH

Due to the high efficiency of the TDI engine, the additional heat that the glow plugs provide is not needed until temperatures drop to approximately 48 degrees F. (9 degrees C.). This function is known as preglow. In addition to preglowing, the glow plugs will continue to operate after the engine is started. This is known as afterglowing and is used to reduce engine noise, improve idle quality and reduce hydrocarbon emissions. The afterglow function is enabled after every start, both hot or cold until the specified time has expired or the engine speed exceeds 2,500 rpm. The glow plug system is controlled by the Engine Control Module (ECM) and the Engine Coolant Temperature (ECT) sensor. The period of the preglow depends on the temperature of the engine coolant as determined by the ECT. A dash mounted indicator light signals the driver that preglowing is operating. Preglowing is not linked to operation of the drivers s door as in some earlier Volkswagen diesel systems.

ALL PREVIOUS NONTDI VW DIESEL ENGINES:
VW ENGINE CODES - ACC, AAZ

The glow plug system is controlled by the automatic glow plug time control module and the glow plug relay. The system is activated by the drivers s door switch. Once the door is opened and then closed, the pre-glow is initiated. The length of time that the system operates is controlled by an engine temperature sensor. Above certain temperature, the system does not operate.

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Old August 6th, 1999, 06:28   #13
SkyPup
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Default Re: Early morning engine bucking or lurching solved!

I have two TDI's and have never had any bucking problems as described. The afterglow setup is to keep the combustion chamber hot enough to support compression ignition since whilst the engine is realatively cold on start-up, the heat generated first goes into the cool cylinder walls instead of into providing heat of combustion, it also improves emissions condsiderably too.
I would tend to believe that this bucking could be attributable to clogged or malfunctioning injectors as the ECU reroutes fuel immediately to the other cylinders to makeup for a loss in a cylinder. In fact, disconnect one of the injectors totally and NO CHANGE in idle or operation is detectable! The ECU can support a 25% loss in power and correct for it internally.
Both my TDI's, when I start up first thing in the morning, I idle in 1st, 2nd, and 3rd the two mile ride I have to my front gate, thereby allowing the engine to slowly warm up. During this time, winter, spring or summer, there has never been any bucking or stalling whatsoever, so that indicates that indeed this situation is problematic and not normal. Once I have idled out the driveway and passed the gate, I run the engine normally. Even idling in any gear below 4th, never have experienced any difficulty.
Hope that helps somewhat
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Old August 6th, 1999, 08:25   #14
Russell Parr
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Location: Atlanta, GA USA
Fuel Economy: 55 mpg Hwy
Default Re: Early morning engine bucking or lurching solved!

I think the real issue here is a thermal one. A "cold" diesel engine, at whatever temperature is at a small disadvantage in terms of combustion. The inherant nature of using physics to burn a substance introduced into super compressed air (around 1800 degrees) will rise as more heat can be kept inside the combustion chamber. At 0 degrees or 100 degrees outside temperature, the engine needs some room to get the combustion temp to a level where the fuel burns without much fuss. Heat flows to a cold source, so until the cylinder walls and surrounding metal surfaces are at or near combustion temp, the engine will be at a thermal disadvantage. Fuel quality and cetane levels can assist this thermal problem by being easier to burn. Remember cetane increases the fuels susciptible to burning, octane in gas slows down the rate at which fuel will burn.

So those of you who do not experience this bucking might have fuel with better cetane rating or possibly better compression etc. Many factors probably contribute to this lurching and sinse every engine is not exactly the same as the next one, this could be a compound result. It might be that a few psi difference in an engine that bucks and one that does not could make the difference. If the compression ratio for one car measured is 18.5 and another is 19.2 there will be a sight increase in the temperature at the maximum compression stroke. Say 1800 degrees compared to 1897 degrees.(just relation example) This might be enough to allow the combustion process to go almost unnoticed.

The bottom line is if all systems check out ok, that bucking, while not normal is probaly more fuel cured than anything else. VW telling people that this is normal is a COP OUT period!

Chow
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Old August 6th, 1999, 20:08   #15
dparnell
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Default Re: Early morning engine bucking or lurching solved!

Someone asked if Dr Diesel raises cetane---it really doesn't. Most rough running of diesels on start up is due to low ambient temperature which make the oil thicker when using a single wt oil. When the engine warms up this goes away. If you are already using a multigrade 5w-something, then I don't know---I run into this problem with diesels using a straight 30wt.

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