Hesitation and one case of eliminating it.
Greetings,
This is my first post on this forum and I am writing this text as repayment for the help MB forums have given me in the past...
My good friend bought a 260E that had sat for several years due to a terrible hesitation issue that several shops had tried to resolve by sucking cash out of the owners billfold.
Here is a brief history of parts replacements and adjustments -
EHA valve replaced.
EHA valve adjusted 1/4 turn.
New fuel distributor.
New cap, rotor, wires and plugs.
Fuel pump.
Computer.
Coolant sensor.
Idle Valve.
According to the previous owner the only aforementioned item that seemed to improve things was the EHA adjustment. Improved, not eliminated.
I started out by following a methodical process of elimination. I first performed a compression test and found everything OK. However when I measured vacuum, my needle was bouncing around 16 inches; a sticky valve. I added Rislone engine cleaner (basically ATF fluid) and drove the car for 100 hard driving miles. When I retested vacuum the needle was rock solid on 16 inches, which to me was OK but perhaps a little low. I now knew that the engine was solid and was then going to poke around the injection system.
Fuel pump delivery test - removed feed line to the fuel distributor and placed the line into a container. I jumpered the fuel pump relay socket and ran the fuel pump for one minute. Fuel flow minimum is 0.5 Gallon a minute, or 1.8927059 liters. This car delivered nearly one Gallon. OK.
Total system fuel pressure test - I went to a breakers yard and cut off several steel lines to be used to test the fuel distributor as the fitting are not common on these cars. I then inserted a pressure gauge between the feed line and my "test fitting", and then connected everything back onto the distributor. Again I jumpered the fuel pump socket and measure a total sytem pressure of 82 PSI, or 5.65517 Bar. 75 to 82 is correct. OK.
System pressure hold test - This is observed immediately after turning off the fuel pump on the total system pressure test. When the pump stops watch your pressure gauge, it should hold 70% of total system pressure for several minutes. This car immediately dropped to zero. Not OK.
Pressure regulator leak down test - Ok, this car failed the system pressure hold test, so I clamped shut the fuel return hose from the regulator and repeated the pressure hold test. It again returned to Zero after the pump was shut off. I then depressed the AFM plate and peered inside the AFM to see if fuel was leaking inside; it was not. This told me that the fuel pump check valve was shot and that the accumulator by the pump was no longer working. These items help make restarting a warm engine nice and crisp (as long as the injectors don't leak) but these will not cause the engine hesitation.
Injector leakdown test - I removed the cold start fuel line from the distributor and inserted my junk yard fitting I spoke of earlier and connected a pressure gauge. I started the car and immediately turn it back off. According to the information I had this pressure should hold at 38 psi, which it did for several seconds then slowly began to drop; leaking injectors.
Injector delivery test - I removed each injector, bent the fuel lines back, screwed the injectors back on, pulled the lines back far enough that I could place small containers under each, placed a pen inside the AFM (to depress the plate) and jumpered the fuel pump socket for 30 seconds. During this test I observed the spay patterns for each injector, then after turning off the pump measured the fuel in each container. They were all within 10% of each other, which is correct. This test does not check cracking pressure which is 41psi. That requires a test jig which I don't have. I was not satisfied with the spray pattern so I took ScotchBrite Red, and I actually scoured the injector end until the tips were clean (I stuck the ScotchBrite in the end and simply twisted the injector back and forth.). I then took a pencil rubber and placed it on the tiny valve on the tip and lapped them (twist back and forth). Guess what? The injectors then sprayed nicely and even had that high pitch squeal you want to hear! Lucky me.
Differential pressure test - I am a little confused on how to configure the pressure gauges to perform this test so I did not perform this test. I was not convinced that the distributor was the culprit so I continued.
Ok, it seemed that everything mechanical was in good working order, yet this car acted like a carburetted engine with a bad accelerator pump. I was convinced that there was a vacuum leak or a bad EHA signal from the ECU. I decided to save the computer testing for last, and even though my vacuum leak spray test revealed nothing, I decide to remove the AFM assembly and the Idle valve hoses for a proper inspection. A quick look at the Idle valve hoses showed no signs of problems.
I dismantled the AFM, first removing the distributor, then removing the lower rubber half of the AFM. Everything looked OK at first so I began to hand clean and inspect the plate mechanism and cleaned everything. I re-centered the plate so that .002" feeler could easily feel around the plate and housing. Everything seemed perfect and I was starting to feel beaten down, thats when I looked at the the rubber lower half of the AFM. It had oily residue on the rubber sealing lip which told me it was leaking! Not using silicone as it does not hold up to fuel, I put a small bead of gasket eliminator on the clean rubber lip and re-assembled the AFM. Then when I was about ready to reinstall the Air valve hoses I decided to check those again. The Hose that the cold start valve goes into was soft and still in good shape, however the hose that goes from the air valve to the AFM had hardened and was loose on the connecting ends. A closer look at the end that attached to the valve revealed small splits; I did not see them at first but when I slightly smashed the hose with my fingers they showed their ugliness (Splits). For under 16$ US dollars I replaced the air valve hose.
Before I reassembled everything I remembered that the EHA valve had been increased by 1/4 turn so I returned it to "stock". Upon re-assembly I started the engine which started right up and ran smoothly. I checked the AFM potentiometer voltage (.76V) and I set the mixture CO and went out for a test run. As soon as I mashed on the accelerator I knew the problem was resolved, and it was!
I must add that I cleaned the main throttle valve and checked all vacuum lines. I believe that the AFM "Boot" and the air valve hose were leaking, and that the injectors were not 100%. But my cleaning and a tank of MOLY LUBE valve cleaner fixed that.
All of this just proves that simple spraying for leaks is not a proper method, and that the only true way to check things is to take it apart, scrutinize, and test. I am grateful that I saved the electronic testing for last. I hope that this experience will help others in this difficult and misleading problem that plagues so many owners.
Cheers,
Blake Dodson
, this car is having none of it, same problem. Where do I go from here guys?
