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#1 Old 10-23-2009
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Fuel Pressure test questions

Hi, I have a 1998 SLK230....I have recently done a fuel pressure check but, am unable to varify the pressures because I cant find them posted anywhere. If any of you can help, Id appreciate it....

I connected a pressure gauge to the Schrader fitting on the fuel rail

1) Turned on the ignition (didnt start the car), pressure went to 48psi
2) Started the car, pressure at idle was 48psi
3) Reved the engine, pressure stayed steady at 48psi
4) Ran the engine at part throttle, pressure stayed steady at 48psi
5) Engine idling, I pulled the vacuum hose from the pressure regulator, pressure went to 58psi.
6) Reconnected the vacuum hose, pressure went back to 48psi

Vacuum at the regulator (at idle) was about 14-15
Vacuum decreased (to about 7) when I reved the engine

My questions are this:

1) Is 48psi within specifications?
2) Is the fuel regulator bad because it doesnt increase fuel pressure during low vacuum (while I was reving the engine)?

Thanks....
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#2 Old 10-23-2009
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Sounds like you have a constant pressure system - quite normal.






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#3 Old 10-23-2009
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Hi Woolly, thanks for the reply....
I understand what you are saying...., but isnt the reason for the vacuum to adjust the fuel pressure under load?
When I reved the engine, the pressure didnt change and I guess I would have expected a spike then settling back to the standard pressure.

do you happen to know what the standard pressure should be?

Thanks
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#4 Old 10-23-2009
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There are two types, some adjust rail pressure with intake pressure and some dont. The ECU is mapped one or the other , as woolly said you seem to have constant pressure BUT its function is when the manifold is under pressure ie boost not vac. So you need to look at the gauge while driving under boost to see if rail pressure goes up.
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#5 Old 10-23-2009
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There in lies my question...which one do I have? Which one does the 1998 SLK230 have?....I can only assume,...thats why I was asking the question and hoping to get the manufactures pressure specifications.
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#6 Old 10-24-2009
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Quote:
Originally Posted by Legin View Post
So you need to look at the gauge while driving under boost to see if rail pressure goes up.
This will tell you what you have. Not sure why you want to know if your engine is running fine. 48psi looks fine based on pressures I have seen when mapping
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#7 Old 10-24-2009
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Hi Legin, the reason Im running these tests is because the engine isnt running fine...and thats a long story in itself

I was hoping someone might know based on personel testing or manufacture documentation.

Thanks for your confirmation on the pressures
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#8 Old 10-24-2009
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Ok I looked at the factory spec for you assuming you have the 111 engine

with vac 3.2 -3.6 barg
without vac 3.7 - 4.2 barg
tickover should hold > 2.5barg for at least 30secs

looking at your numbers its not your problem. I had a very odd problem on my r129 v6 which was intermittent loss of cylinders, it was water droplets in the rail which i dried out in the oven.
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#9 Old 10-25-2009
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Hi Legin, thanks for these numbers, you assumed corrcetly on the engine....If I understand you correctly

@ idle without vacuum hose attached to the FPR
51-58psi (approximately)
@ idle with vacuum hose attached to the FPR
44-50psi (approximately)

Based on these numbers it appears my numbers are within specs.

The tickover statement, I dont understand what this means and I think its the key part to the equation....because my whole assupmtion is that the fuel pressure increases based on vacuum pressure....and I think what you are saying is that there is an increase.

These odd problems are the ones that drive us nuts...I was hoping someone had the same problem I have, but it figures Im the only one. My problem is intermittent hesitation (bog down) during part throttle (cruising). Kinda like the car is going to shift, but then doesnt....and of course it doesnt throw a code....
I appreciate your help...I dont have a manual so I dont know if what I see is in specs or even meaninful.
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#10 Old 10-25-2009
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Shot in the dark but the MAF seems to be the route of a lot of problems on r170 units plus they often dont throw a code I have found this on other vehicles. Can you get a friendly co owner to let you try their MAF ? The only other culprit is corrosion on the connectors especially as its intermitent so check these as well.
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#11 Old 10-25-2009
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Hi Legin, thanks for the suggestion, but the MAF was the first thing I replaced....well, maybe the second, spark plugs were the first.

Can you elaborate on the tickover statement you made? Im very interested in understanding that better...
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#12 Old 10-25-2009
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Its all it says so Im non the wiser. It could be a leakdown rate ie switch of at idle and the pressure should be >2.5bar at 30secs ?
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#13 Old 10-25-2009
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I found it on the internet.....

tickover:
When a petrol engine runs on closed throttle in neutral gear...In other words idling....

Now, thats interesting cause my idle pressure is 48psi @ idle not 35psi

Well, I ordered a new FPR...it arrives Tuesday. Ill rerun my tests with the new valve and hopefully it will be differnt (and better) than the one I have
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#14 Old 10-25-2009
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Mate, With a higher pressure all you can get is overfueling not the lean hesitation you are experiencing, the factory spec is greater than 2.5 bar which you have. I really dont think you have a fuel pressure problem
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#15 Old 10-26-2009
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Hi Legin, thats my point exactly....I believe that the fuel pressure varies between 44-60psi during engine load. This pressure is varied by manifold vacuum...remember I said I believe.
My FPR holds the fuel pressure at a constant 48psi no matter what the engine load is, which is the low end and I believe its causing my fuel starvation issues. (at least I hope)
Its interesting too, that since I removed the vacuum from the FPR (hence the constant pressure is now 58psi) I havent experienced the normal recurring hesitation issue, though there is some slight rough running at times,....I have my fingers crossed that when the new FPR comes tomorrow things will be perfect.
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#16 Old 10-28-2009
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Ive installed the new FPR and have come up with the following results:

1) Turned on the ignition (didnt start the car),
pressure went to (old) 48psi (new) 50psi
2) Started the car, pressure at idle was (old) 48psi (new) 50psi
3) Reved the engine, pressure stayed steady (old) 48psi (new) pressure varied..increased during throttle push and decreased during throttle release.
4) Ran the engine at part throttle, pressure stayed steady (old) 48psi (new) increased from 50psi to 53psi, but settled back to 51psi
5) Engine idling, I pulled the vacuum hose from the pressure regulator, pressure went to (old) 58psi (new) 60psi.
6) Reconnected the vacuum hose, pressure went back to (old) 48psi (new) 50psi

So it seems the new FPR has established a 50psi pressure base over the old 48psi and also varies fuel pressure based on manifold vacuum.

Id love to declair victory and say this has solved the nagging hesitation problem, but I want to drive it a few more days first...let me say this, preliminary driving has been good, so I am optimistic.
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#17 Old 10-28-2009
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I really didnt think this was the issue but happy to be proved wrong. Hope it stays good for you.
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#18 Old 10-28-2009
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Hi Legin, Im not convinced yet either, but seeing the new part work differently and the drive go smoother gives me hope.
Ill know for sure next week....
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#19 Old 01-03-2012
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FPR replacement

Nine:
I have a 99 SLK 230 and have been having problems with hesitation. I was wondering whether your feul pressure regulator replacement ended up being the cure for you. Your last post indicated it was driving better but you wanted to test it further. Any new findings?
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#20 Old 01-03-2012
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kjohns, fyi

this thread is over 2 years old so u might not get a response


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