The OFFICIAL bogging, hesitating, no acceleration from idle thread

[dmurray14]

I've given up. So frustrated with this thing. I may try the cats, we'll see. Either way, it's going to the mechanic and I'm going to tell him to just fix it. I can get my bike up to speed faster than my damn car..

Dan

[flip213]

ok yes it does apply to 94 cars because they are still obd 1. and the return line of the fuel system is underneath the the intake manifold. you can se it from the side near the dipstick. you will see 2 lines, one goes to the fuel filter which is covered by a plate the other one the goes down beside the fuel filter is the return line. all u have to do is pinch it with vice grips. and by the wya everyone cats have nothing to do with hesitation if they are hot or cold if there is a problem the O2 sensor would pic it up. to pinch the line first run the car for about a minute then pinch it with vice grips, the line isnt hard to get at i did it took me 5 min to do it in the dark. and guys it really helps if you have the bentley service manual to test out the parts.

[dmurray14]

ok yes it does apply to 94 cars because they are still obd 1. and the return line of the fuel system is underneath the the intake manifold. you can se it from the side near the dipstick. you will see 2 lines, one goes to the fuel filter which is covered by a plate the other one the goes down beside the fuel filter is the return line. all u have to do is pinch it with vice grips. and by the wya everyone cats have nothing to do with hesitation if they are hot or cold if there is a problem the O2 sensor would pic it up. to pinch the line first run the car for about a minute then pinch it with vice grips, the line isnt hard to get at i did it took me 5 min to do it in the dark. and guys it really helps if you have the bentley service manual to test out the parts.

Thanks for the info - is pinching it going to cause any harm? Did you drive around with it like this?

Also - I don't think the O2 sensor would pick up anything on cars pre-96, since our O2s are BEFORE the cat...so they would have no way of measuring efficiency etc.

Thanks,
Dan

[flip213]

Cross the MAF off the list. New one made no difference...

Getting REAL sick of this damn issue...

Chris - do you plan on trying to pinch the return line? I'd do it if I knew how...

Dan

it wouldnt be the maf because it would show up on the diagnostic and also there is a quick way to see if it works. take off the intake after the maf and run your, once you turn it off look through the maf if u see the hot wire come on after like 4 seconds that means it works. also the car is designed to run on a set program if the maf is disconnected or faulty so when u un plugged it thats why u saw no change.

[flip213]

Thanks for the info - is pinching it going to cause any harm? Did you drive around with it like this?

Also - I don't think the O2 sensor would pick up anything on cars pre-96, since our O2s are BEFORE the cat...so they would have no way of measuring efficiency etc.

Thanks,
Dan

no the bmw guy said pinch it go for the drive test and when your done, to unpinch it.

about the cats just punch them if there is not rattling thats means theyre fine. they couldnt be clogged from debris, they only block if the core is broken. the cats are the first ones in the pipe not the small ones.

[flip213]

ive brought my car to 3 different mechanics about my problem and ide say the service manual helps more than them to find problems. it also helps to be studying mechanical engineering. its well worth the purchase of the book.

[Spood]

::sigh::

It seems mine is temperature dependent as well. I hadn't noticed before because I try to drive it really nice until the engine warms up. I got on it a bit more today and as far as I can tell, it does not do the hesitation thing when still cold. I will try again tomorrow to double check.
In Honda world, VTEC does not do anything until the engine warms up, does the same hold true for VANOS in BMW world?
Flip: What exactly does the fuel return line do? (Aside from return fuel ) I mean, like, what is the process...for lack of better phrasing?

[flip213]

ok first of all vtec prolongs the lifters and valves according with the oil pressure in the motor. vanos has sorta of a clutch on the cams that adjust the timming of the cams at higher rpms.

what the return line does is allows the gas to return to the tank when it is not needed under low loading. the check valve is on that line which maintains the residual preesure in the line, which keeps the pressure in the line when the car isnt running. but when the car is running it prevents vapor lock which happens when the motor is hot. what happens is sometimes the gas will get warm enough to vaporize in the line and when this happens the car isnt getting the right fuel/air mixture its getting more air so it runs lean. the gas turns to air at low pressure like when your car is idling and when u go to give it gas the air passes throught the injectors causing bogging. the check valve also prevents a vacuum in the line when the motor is under load.

[flip213]

my problem was also temp dependant beacause it only did it when the engine was warm. when it was cold no problems at all.

[Spood]

So wait, did pinching the line work for you then?
(Nice explanation BTW...I like when people can give good detailed explanations like that.)

[dmurray14]

it wouldnt be the maf because it would show up on the diagnostic and also there is a quick way to see if it works. take off the intake after the maf and run your, once you turn it off look through the maf if u see the hot wire come on after like 4 seconds that means it works. also the car is designed to run on a set program if the maf is disconnected or faulty so when u un plugged it thats why u saw no change.

A) It only shows up on the diagnostics when the maf isn't PRESENT....
B) Only 92's were hotwire. Ours are hotfilm = no glowing wire or burnoff period.
C) When I disconnected my MAF, the car ran completely different, not the same.

[dmurray14]

ok first of all vtec prolongs the lifters and valves according with the oil pressure in the motor. vanos has sorta of a clutch on the cams that adjust the timming of the cams at higher rpms.

what the return line does is allows the gas to return to the tank when it is not needed under low loading. the check valve is on that line which maintains the residual preesure in the line, which keeps the pressure in the line when the car isnt running. but when the car is running it prevents vapor lock which happens when the motor is hot. what happens is sometimes the gas will get warm enough to vaporize in the line and when this happens the car isnt getting the right fuel/air mixture its getting more air so it runs lean. the gas turns to air at low pressure like when your car is idling and when u go to give it gas the air passes throught the injectors causing bogging. the check valve also prevents a vacuum in the line when the motor is under load.

I'll give it a try...

[Spood]

Does the fuel return line/check valve thing also explain the ~2500RPM shift in performance?

[flip213]

down

[flip213]

i dont know about the shift performance. on mine after i pinched it, the problem went away. so now i just have to go buy the fuel pump where the check valve was. and dmurray14 if you had ur checked with the diagnostic with the maf pluged and nothing showed up it means the maf is fine. any kind of sensor problem will show on the diagnostic them bmw mechanic said that was for sure.

[mr.mike36]

To add a FI Temp Sensor check out Pelican Parts - their summary matches ideally as well. Also in relation to CAT temp, I dont think its the temp of the CAT that is the problem though as it plugs the temps will be hotter and less efficient. I think that the hot CAT is causing issues with all of the above...

Tell me if I am wrong... but lets the CAT is holding exhaust as it should and converting to co2 or 02 then as the CAT ages it has to work harder to 'convert' due to lower flow and the same rich fuel it was burning from day one. Now if the flow isn't properly processing at a correct fuel/air mixture it will work harder and get hotter making the remaining components hotter and less efficient as well (cumbustion chamber, headers, o2, and the complete ehaust) couldnt this situation cause a vapor lock or too much back flow? I think that the BMW adjusts well to this environment but the variables are causing sensors to react haywire...sooo why doesnt the person with the most money try it out and let us know I just barely funded my next two weeks of baby essentials and will have to wait for my tax return. I seen how much the CAT cost and let me tell ya, $750 isn't cheap. What about going to a cheaper universal high flow from like Magna Flow or something? Obviously the BMW CAT didn't last long.

Thanks for everyones input, I think this all makes sense in my puny little brain and it would explain the failed attempts to replace all of the above.

I can't say its not that cats but I would keep in mind that there is nothing happening after the cats. Exhaust is just passing through, if they are clogged up or somehow bad it would happen at all rpms. Another way to say it, if you dropped hte muffler and cat mid section and went for a drive, it would probably perform the same (can't say for sure, there's always exceptions). There are even more sensors that haven't been talked about + ones that have:
Trottle position sensor
Camshaft position sensor
Engine temperature sensor
Coolant temperature sensor
another temp sensor but for the intake manifold
Knock (ping) sensor
Crank position sensor

One other thing, I believe bad grounds (electrical) play roles in performance.

[shortly99]

FWIW my 94 318i (M42) does exactly the same ie bogs in hot weather or when through idling the engine bay gets hot. Brilliant in cold weather, never a problem.

[Spood]

i dont know about the shift performance. on mine after i pinched it, the problem went away. so now i just have to go buy the fuel pump where the check valve was.

Maybe I should re-phrase my question...
Does the problem being RPM dependent still jive with the check valve theory?

[mr.mike36]

In relation to fuel return, it seems as though you are forcing the car to run rich by pinching? If the car reacts to this change are you saying the fuel return check valve is bad or the pump? I am confused with this one...also, isn't vapor lock preety difficult to acheive on intake - I thought this would only be caused by manifold pressure or too lean of a jetting ie too little fuel, too much air intake. Now if you are saying the check valve is bad then gas is just bypassing the injectors all together and recollecting via the fuel return? I need help on this one, trying to process it but having a hard time understanding how this will effect performance through only the mid-RPM range under load...I have no mid-range powerband - only performance is between 3500-5000 RPM with moderate bogging but no decrease in horsepower - acts like a slight fuel starvation which would make sense but...

[Tim Poole]

i've now changed my Valve Cover gasket which sorted out my misfire as it was dumping oil ontop of spark plug number 4 causing it to ark out.

However

I still have an idle problem some days its fine and other days its not, i lift off the throttle and knock it out of gear and it idles and revs by it self between 1.5k Rpm and 3.5k Rpm it will continue to do this even if you leave it and touch nothing

BUT

If you rev the car up past 5k Rpm it will drop back down and idle fine at 1K Rpm

One thing i have noticed is i have no carbon canister

My Fuel tank Breather Valve is currently connected directly to the return breather pipe which should be connected to the Carbon canistor

I also spotted im missing the Protection Grid in my MAF which i assum breaks down the air before entering the MAF

i'm going to get a carbon canister & protection grid and see if the problem continues - ill keep you up to date

will try the return fuel hose clamping too

[wooptard]

I came here for the same reason.. printed all of this out, what a great community!

[dmurray14]

To add a FI Temp Sensor check out Pelican Parts - their summary matches ideally as well. Also in relation to CAT temp, I dont think its the temp of the CAT that is the problem though as it plugs the temps will be hotter and less efficient. I think that the hot CAT is causing issues with all of the above...

Tell me if I am wrong... but lets the CAT is holding exhaust as it should and converting to co2 or 02 then as the CAT ages it has to work harder to 'convert' due to lower flow and the same rich fuel it was burning from day one. Now if the flow isn't properly processing at a correct fuel/air mixture it will work harder and get hotter making the remaining components hotter and less efficient as well (cumbustion chamber, headers, o2, and the complete ehaust) couldnt this situation cause a vapor lock or too much back flow? I think that the BMW adjusts well to this environment but the variables are causing sensors to react haywire...sooo why doesnt the person with the most money try it out and let us know I just barely funded my next two weeks of baby essentials and will have to wait for my tax return. I seen how much the CAT cost and let me tell ya, $750 isn't cheap. What about going to a cheaper universal high flow from like Magna Flow or something? Obviously the BMW CAT didn't last long.

Thanks for everyones input, I think this all makes sense in my puny little brain and it would explain the failed attempts to replace all of the above.

Yes, and I think if the cat's are bad they'll cause it to run rich, pissing off the O2 sesnor as well. It's a theory...

I'd be game for getting new cats if someone can take their cats off, warm up to operating temp, and see if the bogging is still there....

[sidvicious]

FWIW my 94 318i (M42) does exactly the same ie bogs in hot weather or when through idling the engine bay gets hot. Brilliant in cold weather, never a problem.

i'm no terriffic mechanic by any measure, rather, a simple diy wannabe.

does ambient air temperature as opposed to engine temperature play a factor here?

from reading most of the posts, many suggest that ambient air temperature is a factor.

if so, wouldn't that suggest something is wrong on the intake end, or in the actual combustion, as opposed to the exhaust?

i know some basics, like colder air contains more O2 per cubic inch than warmer air. air density, i think its called.

just a thought - now, back to work for me. later guys.

sv-

[dmurray14]

i'm no terriffic mechanic by any measure, rather, a simple diy wannabe.

does ambient air temperature as opposed to engine temperature play a factor here?

from reading most of the posts, many suggest that ambient air temperature is a factor.

if so, wouldn't that suggest something is wrong on the intake end, or in the actual combustion, as opposed to the exhaust?

i know some basics, like colder air contains more O2 per cubic inch than warmer air. air density, i think its called.

just a thought - now, back to work for me. later guys.

sv-

Nope- I think all of our problems are related to engine temperature, not ambient...

[318is4life]

im new to this so whats VANOS?