94 C280 Engine Stumbles - Fuel, Vacuum, or Electrical??? - PeachParts Mercedes-Benz Forum

bbogner12 · #1 06-27-2011, 10:54 PM

If you've read any of my previous posts you'll find I bought a basket case that didn't run. After owning the car for a month the car now is back to running form. It starts, runs, and drives. The problem is I've got something causing the engine to hesitate when pressing the accelerator.

The car starts from a cold start and sounds great. Shortly after starting the car begins to surge. It will fluctuate between 500RPMs and 1200RPMs. I've checked for vacuum leaks and all hoses appear to be connected. Once the car is warm, it seems like the idle levels out and stays around 1000RPMs. Placing the car in reverse or drive, the RPMs fall to 600RPMs.

This is where things get weird. As you try and rev the accelerator (while sitting in park), it will stutter and backfire. It will eventually pickup and then rev properly. When I'm heading down the road I can run the car up and then once the RPMs fall it will hesitate before picking back up acceleration. It doesn't happen every time, but it is pretty consistent.

Is this due to bad fuel? Possibly fuel pressure regulation issue? The question is, what should appropriate fuel pressure be at the rail? It could also possibly be the fuel filter...

Another option is it could be a crack in one of the vacuum lines. Any suggestions on how to test this theory without replacing each line?

The third is there are possible wire with insulation issues. While I don't think this is it the issue, I'm not ruling it out since I did replace the harness for the ABS/ASR system. Which means it is possible other parts of the harness are causing a problem. Upon inspection I didn't see anything.

Any help is appreciated.

Brian

mbdoc · #2 06-28-2011, 08:21 AM

How old is the FUEL?

The fuel pressure (regulator) & air mass sensor are responsible for over 95% of fuel mixture correction.

bbogner12 · #3 06-28-2011, 12:41 PM

As I bought the car non-running, I don't have an accurate picture on timeline. I'd estimate that the gas is 13 months old. There were only a couple gallons in the tank once I was able to start it as the low fuel light was on. I went ahead and added 5 gallons of fresh gas to the tank.

If I test for fuel pressure at the rail, and it is right on, wouldn't that tell me if it is regulating fuel properly? Which would lead me back to a vacuum issue which means I need to replace the vacuum lines. Can you tell me what the pressure should read at the fuel rail? If you don't think this could be the culprit I'm open to suggestions. I'm also not opposed to draining the gas completely or just adding another 5-10 gallons to the tank since it is a 16 gallon tank. That should dillute the old gas enough to eliminate that question.

mbdoc · #4 06-28-2011, 05:02 PM

Fill the tank with 92RON & then check fuel pressure, 55-60 psi.

bbogner12 · #5 06-28-2011, 07:41 PM

Thanks for the response. I'll go get more fuel for it tomorrow. Here is what I have based off the pressure test I ran tonight:

I went to Autozone on my way home and picked up their "loan a tool" the Actron Fuel Pressure Tester and according to the book, it says Fuel Pump Pressure should run between 46-52PSI.

With gauge connected and key in the "On" position the fuel pressure read 52PSI. As I left the key on the pressure dropped and stayed steady at 45PSI.

When I started the car, the pressure read 51-52PSI. As the engine began to chug, the fuel pressure rose to 58PSI.

As I said in the earlier post, the RPMs fluctuated. As they rose and fell, the pressure varied from 50-54PSI. Once the engine warmed, the RPMs leveled out and it began to idle around 700RPMs, the engine held steady at 52PSI.

From my analysis it isn't the fuel delivery unless it is at the injector level but it should exhibit other behaviors (unless bad O rings). Which leads me to either bad gas or faulty vacuum lines. Please let me know any additional thoughts. I'll post again once I have a full tank.

Thanks!
Brian

mbdoc · #6 06-29-2011, 08:20 AM

Those pressure readings are very close, so that is not the problem.

bbogner12 · #7 07-01-2011, 01:37 PM

I went ahead and drained the gas tank completely. I then added new 94 octane gas to the car. I'm still getting fluctuating RPMs upon initial startup. Once the car does warm up, it sound pretty good but still produces an occassional backfire at acceleration.

I took it out today around a 5 mile loop and it was doing great until I put the pedal down off a stop (3 miles into the route). It spun up very well to 60MPH. I let my foot off the gas and let it decelerate on its own. As I lightly applied my foot to the gas, it hesitated and backfired around 40MPH.

Not sure what is causing the issue. Thoughts?

Brian

bbogner12 · #8 07-03-2011, 07:34 AM

For those who might read this post in the future, my backfire issue has seemed to clear up. I believe this was caused by condensation built up in the exhaust system from sitting for greater than a year. While the gas may have been a contributing factor. We'll never know because that gas will not be used in this vehicle.

I decided it was time to take it out on the open road. I took it out on the interstate where it handled beautifully. I then took a drive through town where it never once backfired. I've got a few new parts coming, primarily cosmetic at this point, a new radio, headliner material, cabin filter, air filter, and new front brakes. I've also got to take apart the dashboard to fix my center vents (that don't work)...I found a pretty good tutorial on that http://kropf.net/w202/Dashboard.html

Brian