slight miss at cruise speeds...any ideas???

[Robert K]

I have what appears to be a slight miss at cruising speeds in my 1991 535i. It typically occurs from 55-70 mph at steady throttle on the highway. It just feels that the engine is not pulling smooth and evenly. There's a slight jerking feel to it. The engine has new plugs, a new fuel filter and a recent valve adjustment to .013 cold. I also replaced a couple of vacuum hoses after finding a couple of the originals cracked. At first I suspected vacuum leaks. Once I found the cracked hoses, I thought I had it. But the condition persists.

I'm figuring it's either a fuel or ignition related problem. But the fact that full throttle produces smooth, hard acceleration has me baffled. I checked the plug wires for leaks one night (at least the portion of wires not covered by the plastic loom) by viewing the running engine in total darknexx. I didn't really see anything or hear any snapping noises. The plug wires are the originals, so it may be time to replace them anyway. I also checked the cap and rotor and they appear to be fine.

I also suspect the fuel injection system. I'm sure I'm still running the original pump and regulator. But again, the fact that the car accelerates fine under full throttle has me stumped. I would think a pressure problem or injector problem would show up at full thottle for sure.

I haven't checked compression yet, but may try to this weekend. If I can track down a fuel pressure gauge, I'd like to go through the fuel system diagnostic procedure too. But this problem is driving me nuts. Any other thoughts would be greatly appreciated.

Grace and peace,

Robert K
1991 535i

[ryan roopnarine]

as your load gets lighter, the vacuum is supposed to increase. i believe that in the 1989 535 review by C&D said that your car only needed 15 hp to do 55 and 35 to do 70 mph, so you would be under relatively little vacuum compared to WOT acceleration. I would think that maybe there's a cracked hose somewhere that you haven't found yet. do you have any idle problems? ill go find the link that i showed to warren N about fuel regulators from sean and johan's site.


I have what appears to be a slight miss at cruising speeds in my 1991 535i. It typically occurs from 55-70 mph at steady throttle on the highway. It just feels that the engine is not pulling smooth and evenly. There's a slight jerking feel to it. The engine has new plugs, a new fuel filter and a recent valve adjustment to .013 cold. I also replaced a couple of vacuum hoses after finding a couple of the originals cracked. At first I suspected vacuum leaks. Once I found the cracked hoses, I thought I had it. But the condition persists.

I'm figuring it's either a fuel or ignition related problem. But the fact that full throttle produces smooth, hard acceleration has me baffled. I checked the plug wires for leaks one night (at least the portion of wires not covered by the plastic loom) by viewing the running engine in total darknexx. I didn't really see anything or hear any snapping noises. The plug wires are the originals, so it may be time to replace them anyway. I also checked the cap and rotor and they appear to be fine.

I also suspect the fuel injection system. I'm sure I'm still running the original pump and regulator. But again, the fact that the car accelerates fine under full throttle has me stumped. I would think a pressure problem or injector problem would show up at full thottle for sure.

I haven't checked compression yet, but may try to this weekend. If I can track down a fuel pressure gauge, I'd like to go through the fuel system diagnostic procedure too. But this problem is driving me nuts. Any other thoughts would be greatly appreciated.

Grace and peace,

Robert K
1991 535i

[ryan roopnarine]

if you haven't read it yet, here it is

http://bmwe32.student.utwente.nl/sean750/fuel_pressure_regulators/FuelPressureRegulators.htm

there's some other stuff there that might help you, too. my apologies if you've seen this before.

off to drink a germanian beer and pat myself on the back about my diff oil some more.

[Bill R.]

I have what appears to be a slight miss at cruising speeds in my 1991 535i. It typically occurs from 55-70 mph at steady throttle on the highway. It just feels that the engine is not pulling smooth and evenly. There's a slight jerking feel to it. The engine has new plugs, a new fuel filter and a recent valve adjustment to .013 cold. I also replaced a couple of vacuum hoses after finding a couple of the originals cracked. At first I suspected vacuum leaks. Once I found the cracked hoses, I thought I had it. But the condition persists.

I'm figuring it's either a fuel or ignition related problem. But the fact that full throttle produces smooth, hard acceleration has me baffled. I checked the plug wires for leaks one night (at least the portion of wires not covered by the plastic loom) by viewing the running engine in total darknexx. I didn't really see anything or hear any snapping noises. The plug wires are the originals, so it may be time to replace them anyway. I also checked the cap and rotor and they appear to be fine.

I also suspect the fuel injection system. I'm sure I'm still running the original pump and regulator. But again, the fact that the car accelerates fine under full throttle has me stumped. I would think a pressure problem or injector problem would show up at full thottle for sure.

I haven't checked compression yet, but may try to this weekend. If I can track down a fuel pressure gauge, I'd like to go through the fuel system diagnostic procedure too. But this problem is driving me nuts. Any other thoughts would be greatly appreciated.

Grace and peace,

Robert K
1991 535i

[Bill R.]

start by checking the plugs again, and then check the wires with an ohmeter , then check the cap and rotor... I suspect a misfire there rather than the fuel injection... Or even a loose crank position sensor....
Usually an ignition miss will show up like that under a light load but when you step on the gas it richens it up enough not to miss.... I seriously doubt that there is anything wrong with your compression also.... If everything electrical checks out ok then I would think about the O2 sensor...






I have what appears to be a slight miss at cruising speeds in my 1991 535i. It typically occurs from 55-70 mph at steady throttle on the highway. It just feels that the engine is not pulling smooth and evenly. There's a slight jerking feel to it. The engine has new plugs, a new fuel filter and a recent valve adjustment to .013 cold. I also replaced a couple of vacuum hoses after finding a couple of the originals cracked. At first I suspected vacuum leaks. Once I found the cracked hoses, I thought I had it. But the condition persists.

I'm figuring it's either a fuel or ignition related problem. But the fact that full throttle produces smooth, hard acceleration has me baffled. I checked the plug wires for leaks one night (at least the portion of wires not covered by the plastic loom) by viewing the running engine in total darknexx. I didn't really see anything or hear any snapping noises. The plug wires are the originals, so it may be time to replace them anyway. I also checked the cap and rotor and they appear to be fine.

I also suspect the fuel injection system. I'm sure I'm still running the original pump and regulator. But again, the fact that the car accelerates fine under full throttle has me stumped. I would think a pressure problem or injector problem would show up at full thottle for sure.

I haven't checked compression yet, but may try to this weekend. If I can track down a fuel pressure gauge, I'd like to go through the fuel system diagnostic procedure too. But this problem is driving me nuts. Any other thoughts would be greatly appreciated.

Grace and peace,

Robert K
1991 535i

[George M]

many 1991 535i car's had EML or a fly by wire throttle...may be your issue. I presume you are not getting the EML light indicative of any stored fault code specific to your EML. EML and DME do not share the same diagnostics.
EML codes are only retievable via MODIC diagnostic computer at the dealer or by eqivalently equipped independents. Another avenue to consider...if other areas mentioned above do not lend a solution.
George

[Robert K]

If so, my car does not have that. It has a standard throttle housing with a cable going to the accelerator pedal and another to the cruise control module. Which makes me wonder if I need to check the cruise control module next. I hadn't even thought of that one. This problem shows up with the cruise on, but I can't remember if it does when my foot is controlling the throttle. Another thing to check today. But thanks for the reply George. And guess what??? No leaks on the floor under my car!!! Man, that is wonderful!!! We just moved into this house at the beginning of March. It had a nice, big 2 car garage with clean, polished concrete floor. I had a piece of cardboard and fiberboard under the car to soak up the oil. Now she's sitting on nice clean concrete only.

Grace and peace,

Robert K
1991 535i

[Bill R.]

system bmw calls the gas pedal position sensor the drivers wish sensor ( meaning what you wish for and what you get aren't always the same thing):)










many 1991 535i car's had EML or a fly by wire throttle...may be your issue. I presume you are not getting the EML light indicative of any stored fault code specific to your EML. EML and DME do not share the same diagnostics.
EML codes are only retievable via MODIC diagnostic computer at the dealer or by eqivalently equipped independents. Another avenue to consider...if other areas mentioned above do not lend a solution.
George

[Robert K]

These are brand new plugs Bill. Now that you mention it, I'm not sure I really had this problem before installing the new plugs. I believe I checked to make certain they were the correct part number for my car. At first, I installed them with the factory .028 gap. But then I rememebered everyone suggesting a wider gap, so I opened the gap to .033. What should I be checking in regards to the plugs??? It did suprise me when you stated that ignition problems show up at light throttle, but it makes sense.

I'll check the crank sensor, but it should be okay since I removed it to tighten the crank sensor bracket bolts a few months back when I thought they might be causing my oil leak. I snugged all those bolts up pretty good, but I'll check again anyway.

Also, what does one check with the plug wires? Or for that matter the cap and rotor. I did remove the cap and rotor when this all started. I looked at the contacts and also washed both with dishwashing soap and then dried with a hairdryer before reinstalling. I didn't notice any cracks or carbon trails. As I mentioned, the plug wires are originals. Whether they are the actual problem or not, I probably just need to break down and replace them. With the long runs they have being parallel to each other inside the plastic cover, I'm sure it's very easy for them to leak.

As far as the O2 sensor, it probably is time. I'm at 130,000 and I changed it last around 60,000. I just hate to change parts in an attempt to luck into finding the problem, you know what I mean? On a BMW, that can get expensive.

Thanks for the reply. All additional info is always greatly appreciated.

Grace and peace,

Robert K
1991 535i

[Bill R.]

box.... pull them and look at each one carefully to check for any signs of arcing to ground... or cracks in the porcelain exterior... just inspect the whole plug carefully for any signs that one is not firing the same as the others... Cap and rotor you look for carbon tracking , which washing with detergent may have removed any obvious signs.. and you pull the rotor off and look at the back side of it for any signs that it has been arcing through to the bottom... Check the connector where the coil wire goes into the coil for any signs of corrosion etc... Make sure that all the spark plug wires were securely seated on the plug and making good contact... The resistance on the plug wires is 6k ohms from end to end... Your wire is solid wire with very low resistance, the plug end connector has 5k ohms resistance built into it and the dist cap wire end had 1k ohms built into it.. for a total of 6k on each plug wire regardless of length... Unscrew the connector on the O2 sensor and reinstall it just in case there is any corrosion on the contacts, unplug and replug the connector on the crank sensor and temp sensor, afm etc.. Check the valve for the fuel tank ventilation system... You've already looked for cracks in the bellows between the throttle body and afm but look again for anything that might cause a vacum leak... a vacum leak will also cause a light throttle miss sometimes. Check the vacum hose and fitting that goes to the brake booster.. a bad booster will cause a light throttle miss also... as well as a hard brake pedal... Usually its something real minor that you wouldn't think would be enough to cause a miss but at light throttle settings such as cruise the engine is running lean enough and on the o2 sensor closed loop so that very small changes are enough to make it misfire... The leaner an engine runs the more that everything has to be just right for it to work correctly








These are brand new plugs Bill. Now that you mention it, I'm not sure I really had this problem before installing the new plugs. I believe I checked to make certain they were the correct part number for my car. At first, I installed them with the factory .028 gap. But then I rememebered everyone suggesting a wider gap, so I opened the gap to .033. What should I be checking in regards to the plugs??? It did suprise me when you stated that ignition problems show up at light throttle, but it makes sense.

I'll check the crank sensor, but it should be okay since I removed it to tighten the crank sensor bracket bolts a few months back when I thought they might be causing my oil leak. I snugged all those bolts up pretty good, but I'll check again anyway.

Also, what does one check with the plug wires? Or for that matter the cap and rotor. I did remove the cap and rotor when this all started. I looked at the contacts and also washed both with dishwashing soap and then dried with a hairdryer before reinstalling. I didn't notice any cracks or carbon trails. As I mentioned, the plug wires are originals. Whether they are the actual problem or not, I probably just need to break down and replace them. With the long runs they have being parallel to each other inside the plastic cover, I'm sure it's very easy for them to leak.

As far as the O2 sensor, it probably is time. I'm at 130,000 and I changed it last around 60,000. I just hate to change parts in an attempt to luck into finding the problem, you know what I mean? On a BMW, that can get expensive.

Thanks for the reply. All additional info is always greatly appreciated.

Grace and peace,

Robert K
1991 535i

[Robert K]

Thanks for the detailed info Bill. I hadn't really thought much about how correct things had to be at light throttle for it to run right. But since it runs great at other throttle settings, it probably is something small. I will check the vacuum line to the brake booster. I've not noticed anything different about the pedal, but those 14 year old vacuum lines are definitely suspect for cracks and leaks. I discovered that when I took one of the lines loose to remove the valve cover and it just snapped. That's when I started looking closely at the others. Anyway, hopefully I'll find the problem and have it solved this afternoon. Thanks again for the help.

Grace and peace,

Robert K
1991 535i

[George M]

Robert...it goes without saying that your best effort would be to follow up on what Bill suggested. And Bill...your comment about EML is too funny...a wish peddle indeed.
BMW's early foray into EML in particular with DK motor'ed throttle bodies on the 750iL and later M-30's is a good effort I guess but I thank my lucky stars my M-30 has the more conventional cable throttle and less complexity. It stands to reason however that in the case of pure redundancy with the V12, the DME's could possilby synch each throttle better with this method. Also, EML would work more seemlessly with ASC/traction control where the DME could back off throttle to prevent wheel spin. I personally prefer without though some traction control systems really do work well.
George

[Robert K]

Bill, I checked the plugs, wires, cap and rotor a while ago. Everything looked okay except for one plug. I noticed a slight dark speck near on the porcelin right next to the metal body. I'm going to look through my spare parts to see if I have another plug and try it if I do. If not, I'll get another and see if that's it.

I'm not quite certain, but my O2 connection might have been a bit loose. It didn't seem to take much to turn the collar to get it apart. But I noticed that it turned fairly hard then sort of clicked to lock into place when I put it back together. Had me wondering if it was barely on. We'll see when I take it out for a spin later.

What about a throttle position sensor? I know for a fact that the full throttle switch is not working in mine. I just haven't ordered a replacement. I was wondering if the rheostat portion of the switch could develop a problem. I know on my Tbird, one of those switches developed a dead spot that would kill the engine when it hit it. I think I'm going to go ahead and place an order for that switch and a set of plug wires come Monday. I know the TPS needs replaced and I'm sure the wires could stand to be replaced. The hoses from the manifold to the brake booster are the originals with the crimp type clamps. I'm going to replace those too just because they're old and it's an inexpensive repair. One thing's for certain, by the time I get done replacing and tweaking everything, this sucker ought to get about 30 mpg. I wish!!!

Well, if I still have the problem after trying these repairs, I'll come crying back again. Thanks to you and all others for the suggestions.

Grace and peace,

Robert K
1991 535i