Not to long ago I recently bought a 98 jetta tdi with the 90 hp 1.9, my fuel mileage is way low compared to what it should be, I drive 45miles one way a day to college, I have to be light on the pedal or it absolutely dumps fuel, no mods everything stock on the car. I noticed it wasn’t warming up at all, so I replaced the thermostat, it still only gets up to heat when I hit the 75 mph (by up to heat I mean the gauge will actually start reading coolant heat) I know the egr valve is bad in it, or atleast throwing a code, but I’ve noticed when it does get up to heat it gets better mileage, I’m not hotrodding it cause when I do the gauge goes down a ton, even if I just shoot a gap real quick or across the road real quick, you can literally see the gauge drop. I’m thinking it’s just the egr, but I wanted to know if anyone else has ideas on what it may be. Thanks for the help!!
Bad fuel mileage, please help
- Thread starter Rambutter
- Start date
Steve Addy
Top Post Dawg
On a car of this age and likely with multiple prior owners there's going to be a whole host of things that could contribute to these problems.
The first thing I would do and the thing that will provide the most value would be to find someone with VCDS (vag-com) and see what's going on with problem codes and check to see what your actual water temp is at the engine sensor.
Assuming that you don't have VCDS to rely on, how many miles of 55/60 mph does it take for the engine to get to full operating temperature? In the winter (10-20F outside) my tdi takes 7+ miles of non-stop driving to reach full operating temp even when the grille is covered.
In the situation above is the heater working so the cabin gets warm? If that's happening and the heater air is hot then I would suggest that your coolant temp sensor is bad or the cluster gauge for water temp has a problem. Is the gauge cluster as it was from the factory?
If you replaced the thermostat and it still fails to reach operating temperature then I would definitely find VCDS to determine what the actual temp is.
Failed sensors can cause excessive fuel consumption. It would also help eo know how many miles your car has and whether the last timing belt change was performed by someone who had VCDS to perform the necessary timing adjustments?
Sorry to hear about your problems.
Steve
The first thing I would do and the thing that will provide the most value would be to find someone with VCDS (vag-com) and see what's going on with problem codes and check to see what your actual water temp is at the engine sensor.
Assuming that you don't have VCDS to rely on, how many miles of 55/60 mph does it take for the engine to get to full operating temperature? In the winter (10-20F outside) my tdi takes 7+ miles of non-stop driving to reach full operating temp even when the grille is covered.
In the situation above is the heater working so the cabin gets warm? If that's happening and the heater air is hot then I would suggest that your coolant temp sensor is bad or the cluster gauge for water temp has a problem. Is the gauge cluster as it was from the factory?
If you replaced the thermostat and it still fails to reach operating temperature then I would definitely find VCDS to determine what the actual temp is.
Failed sensors can cause excessive fuel consumption. It would also help eo know how many miles your car has and whether the last timing belt change was performed by someone who had VCDS to perform the necessary timing adjustments?
Sorry to hear about your problems.
Steve
Sweet, I’ll have to look around, the guy I got it from is one of the now three owners, whoever he got it from, he was the original owner, the temp sensor would make sense, I didn’t even think about it though cause it eventually goes up, but I drive about 17 or so miles of 65mpg before the 75, I spaced about cardboarding the grille too, I should definitely try that, the cabin does get warm, but I can go out and touch the engine and it is still freezing to the touch. As it’s getting warmer though I have noticed the temp will stay up, whereas before it would drop as soon as I slow down or hit a light, I’m guessing I just need to cardboard the front I should definitely try that thank you
iluvmydiesels
Veteran Member
idk he's up far north, mileage should be adequate or better. if its low there are several things you can check.
is it possible when you put a new thermostat in you put like an 80C dg or (less?)such in? you have to make sure you get the stock 87C dg thermo. then again it alone doesnt do it. right now i have a problem, and its not a common topic.
check your coolant glow plugs. i take a multi-meter, and there are 3 GPs on the flange coming of the head over the tranny. the hose goes to the heater core, it may go the the egr first if thats still in the water 'line'. start car from cold(then again your not getting much heat, but still--), give it a few seconds and test volts @each GP. probably will be none. now you want to check the coolant GP 'fuse box', i have the same model it seems. its on the firewall, to the left of the brake booster. #1:check fuse, it should be a std GP 50A fuse. if the fuse is intact, again with engine cold, running, check fuse for volts, first on the 'in' side, then the other side of the fuse, the connection. the power out connection should be under this. probably cold.
is it possible when you put a new thermostat in you put like an 80C dg or (less?)such in? you have to make sure you get the stock 87C dg thermo. then again it alone doesnt do it. right now i have a problem, and its not a common topic.
check your coolant glow plugs. i take a multi-meter, and there are 3 GPs on the flange coming of the head over the tranny. the hose goes to the heater core, it may go the the egr first if thats still in the water 'line'. start car from cold(then again your not getting much heat, but still--), give it a few seconds and test volts @each GP. probably will be none. now you want to check the coolant GP 'fuse box', i have the same model it seems. its on the firewall, to the left of the brake booster. #1:check fuse, it should be a std GP 50A fuse. if the fuse is intact, again with engine cold, running, check fuse for volts, first on the 'in' side, then the other side of the fuse, the connection. the power out connection should be under this. probably cold.
iluvmydiesels
Veteran Member
i have a few tests for engine, 1 is a compression test.
an injector pop test, and spray pattern when 'popping'. then again by now plenty of injectors need service. recalibration, usually new injector bodies complete w/nozzles are a better price. bosch.
there are some other things you can do. check inlet line to inj pump for air. if you can still see as is supposed to be a clear line.
use vcds to check pump timing.
an injector pop test, and spray pattern when 'popping'. then again by now plenty of injectors need service. recalibration, usually new injector bodies complete w/nozzles are a better price. bosch.
there are some other things you can do. check inlet line to inj pump for air. if you can still see as is supposed to be a clear line.
use vcds to check pump timing.
Thank you I’ll was looking at injectors for it and I was hoping it’s not the problem, I’m gunna clean the oil off the top of the engine and see if it’s actually leaking or if someone had spilled oil on it, it’s not a very convincing leak being how the oil is ontop of the valve cover, but not much below it, then I’ll try running a injector cleaner through it first before jumping at injectors
.
So I don’t know if I’m understanding this, are you saying there are separate glow plugs for the coolant? I have an engine code for my glow plug rail but I figured the glow plugs themselves are just old, I don’t have much for service records before the car so I figured they had never changed them or had only a couple of times
ToddA1
Top Post Dawg
Coolant glow plugs aren’t monitored. If you have a CEL, it’s the main rail or GPs.
I’ve had luck with just cleaning the corrosion off the contact, and it cleared my code. The harness can be easily made with sone 14ga wire and female bullet connectors.
-Todd
I’ve had luck with just cleaning the corrosion off the contact, and it cleared my code. The harness can be easily made with sone 14ga wire and female bullet connectors.
-Todd
iluvmydiesels
Veteran Member
well you have a used car, you just bought. sounds like it needs at least a few things. some where i have a list for projects like this...oh yea in my head, it may take quite a bit to freshen up just about everything. thats just me, as these cars run good when treated right.
like said, sounds like you just bought this car. so go buy a new fuel filter, and air filter. you also need a can of diesel purge, you ll have to find a store around you that has it in stock, sometimes napa, advanced or autozone may carry it. you need a few items in order to 'bottle feed' cleaner/flush. length of fuel line (i prefer f.i. line), and a connector, you should be able to find in a plumbing shop. fuel line should be 3/8", the connector 3/8" to 3/8".
running a full can straight can have the best results for cleaning injection system. find a place to put the can, can be on the side of the air box, hopefully its kind of secure, take the inlet line off of the fuel filter, insert connector, take a good length of fuel line, put on other side of connector, this one runs all the way to the bottom of the can. take another length of fuel line(you ll cut the line you buy, maybe 2 feet or 3), this line you put on the pump return, and run this also into the can. put the can in a good spot, maybe use some rubber bands to hold it.
i think your ready, you go in, of have your help in the car, and start it. run at different speeds, and at idle, up to about 2000rpms, maybe some more, run the can out. again change engine speeds.
afterwards take lines and connector you put on for flush off, put in a good spot to use in the future, put new fuel filter in, prime, you can do by the return opening, with additive or fuel, or by the inlet, put fuel line on and a funnel and add fuel or additive. dont spill! secure filter, add additive to the inlet line, to prime, dont fill all the way up, connect to filter. notice i didnt say add fuel to inlet line. here it needs to be filtered. put in new air filter. check air box, it may be dirty.
like said, sounds like you just bought this car. so go buy a new fuel filter, and air filter. you also need a can of diesel purge, you ll have to find a store around you that has it in stock, sometimes napa, advanced or autozone may carry it. you need a few items in order to 'bottle feed' cleaner/flush. length of fuel line (i prefer f.i. line), and a connector, you should be able to find in a plumbing shop. fuel line should be 3/8", the connector 3/8" to 3/8".
running a full can straight can have the best results for cleaning injection system. find a place to put the can, can be on the side of the air box, hopefully its kind of secure, take the inlet line off of the fuel filter, insert connector, take a good length of fuel line, put on other side of connector, this one runs all the way to the bottom of the can. take another length of fuel line(you ll cut the line you buy, maybe 2 feet or 3), this line you put on the pump return, and run this also into the can. put the can in a good spot, maybe use some rubber bands to hold it.
i think your ready, you go in, of have your help in the car, and start it. run at different speeds, and at idle, up to about 2000rpms, maybe some more, run the can out. again change engine speeds.
afterwards take lines and connector you put on for flush off, put in a good spot to use in the future, put new fuel filter in, prime, you can do by the return opening, with additive or fuel, or by the inlet, put fuel line on and a funnel and add fuel or additive. dont spill! secure filter, add additive to the inlet line, to prime, dont fill all the way up, connect to filter. notice i didnt say add fuel to inlet line. here it needs to be filtered. put in new air filter. check air box, it may be dirty.
iluvmydiesels
Veteran Member
it seems obvious from what your saying, you bought a car in need of 'work', im hesitant to say a 'project car', but it may be, or about in that need. certainly needs quite a bit of maintenance.
if you look back to my first two posts. post #5 and #6.
post #5 i outlined some basic ways to see if your coolant glow plugs are working. using a multimeter. you may not know they are there. but you have problems with engine heat. this is a good place to check. seems with your problem, you may have more problems.
post #6 i put some trouble checking tests in. for one check the clear inlet fuel line for air.
you say your valve cover has oil on it. it could be the CCV, either not working right, or excessive CC pressure, or blowby. or a leaking CCV line. check the CCV, its the hockey puck on top of the valve cover. the lower CC line may need to be replaced, not uncommon. you can find this line/hose.
if its blowby, and/or excessive CC pressure, a compression test would come first.
for your EGR not working, you can try this. find the EGR controller, it should be in the factory spot. near the air box and fuel filter on the inside of the passengers side fender. the EGR should have 3 (?? i think) vacuum lines coming off of it. one will go behind the timing cover, all the way over the valve cover and to the vacuum pump line to a nipple. the other is i think 5mm or 6mm vac line, it goes from the controller to the EGR valve. if either of these are leaking, you dont want that. they will let dirty air right into the engine, and can also cause CC pressure. (ie you have oil on your valve cover, can be a problem with this). the 3rd vac line goes from controller to the air box. change all these lines, the controller has an electrical connection, i take this apart, use electronic spray and clean up contacts, let dry. reconnect, now try to clear codes, and see if that clears up EGR fault. it may, if so, for this your in luck, if not you have a bigger problem. EGR isnt the most important system. take care of it later. but stop any vacuum leaks is an important first point of business.
if you look back to my first two posts. post #5 and #6.
post #5 i outlined some basic ways to see if your coolant glow plugs are working. using a multimeter. you may not know they are there. but you have problems with engine heat. this is a good place to check. seems with your problem, you may have more problems.
post #6 i put some trouble checking tests in. for one check the clear inlet fuel line for air.
you say your valve cover has oil on it. it could be the CCV, either not working right, or excessive CC pressure, or blowby. or a leaking CCV line. check the CCV, its the hockey puck on top of the valve cover. the lower CC line may need to be replaced, not uncommon. you can find this line/hose.
if its blowby, and/or excessive CC pressure, a compression test would come first.
for your EGR not working, you can try this. find the EGR controller, it should be in the factory spot. near the air box and fuel filter on the inside of the passengers side fender. the EGR should have 3 (?? i think) vacuum lines coming off of it. one will go behind the timing cover, all the way over the valve cover and to the vacuum pump line to a nipple. the other is i think 5mm or 6mm vac line, it goes from the controller to the EGR valve. if either of these are leaking, you dont want that. they will let dirty air right into the engine, and can also cause CC pressure. (ie you have oil on your valve cover, can be a problem with this). the 3rd vac line goes from controller to the air box. change all these lines, the controller has an electrical connection, i take this apart, use electronic spray and clean up contacts, let dry. reconnect, now try to clear codes, and see if that clears up EGR fault. it may, if so, for this your in luck, if not you have a bigger problem. EGR isnt the most important system. take care of it later. but stop any vacuum leaks is an important first point of business.
iluvmydiesels
Veteran Member
from another section of forum.
i had figured this may easily be a consideration of part of your problem.
i think there may have been other things,,, anyways.
like nozzles, i kind of forgot to mention these, however, they are not cheap, and if injectors themselves need calibration,, well i think i hinted toward all new injectors prev.
Joshua4507
Member
So I daily drive my mk3 and have done so for years it is a kick ass car.I took the car from California (where I live) to a wedding in Seattle Washington. On that road trip I was getting over 50 mpg now when I blew threw the Oregon hills thus dropping my fuel millage down to 47mpg but my average for the whole trip was 51mpg. The crazy thing to me is I was doing between 55 and 85 on the innerstate. When I'm at home doing my regular stuff my car gets on average between 33 to 37mpg and that is 95%stop light to stop light with 5% freeway but not for long.
Right now my car is going thru fuel way faster than normal and I suspect that my egr/intake is gotta be pretty nasty. I could be wrong it could be a few other things but won't know for sure till I can get the car in the garage and see what's really going on.
With that said I love my mk3 tdi it's been a great car I've put about 35,000 miles on it. I have another motor harness and ecu that I'm planning on swapping into a Toyota 4x4. That how much I like these vw ahu diesel motors.
Right now my car is going thru fuel way faster than normal and I suspect that my egr/intake is gotta be pretty nasty. I could be wrong it could be a few other things but won't know for sure till I can get the car in the garage and see what's really going on.
With that said I love my mk3 tdi it's been a great car I've put about 35,000 miles on it. I have another motor harness and ecu that I'm planning on swapping into a Toyota 4x4. That how much I like these vw ahu diesel motors.