1978 - 1979 928 CIS hard start problems

[Rich928]

Dr. bob provided a lot of great information regarding the cold start fuel injector in the Fuel Pump Not Priming thread.

I'll address a CIS hot start problem that may be difficult to diagnose. I found this issue with both of my 1979 928s.

The Fuel pressure accumulator is a spring loaded piston in a cylinder that fills with fuel and holds pressure. If there is a pressure loss in the fuel lines the spring loaded piston pushes pressurized fuel into the lines. The two functions of the accumulator are to provide fuel pressure at start-up and to keep pressure in the lines to reduce vapor lock (fuel boiling in the lines when the engine is turned off).

The typical failure mode of the fuel pressure accumulator is the piston seal leaks and fuel drips, as long as the fuel leaks are found (or smelled) the diagnosis is simple. A frozen piston with no leaks is more difficult to diagnose, without the residual pressure hot start problems occur and the most common thought is a fuel pump problem ... however, cold starts are less of a problem thus confusing things.

I replaced the fuel pressure accumulators on both of my 1979 928s. The part numbers today is 928.110.197.31 (old p/ns: 928.110.197.05 and 928.110.197.07).

Those of you who have flown on early Airbus A32X airliners may have heard what has been described as "the barking dog" strange noises prior to pushback. The strange noises are the Power Take-off Unit (PTU) hydraulic pressure accumulator charging. I think the noise has been reduced in newer versions.

Rich
928 OC Membership Chairman
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue soon to be upgraded with "beast" internals
1979 928 Euro 5-liter track beast soon to be cannibalized
2009 Cayenne Base

Fuel pressure accumulator.pdf

(229.91 KiB) Downloaded 17 times

[Tom]

Dr. bob provided a lot of great information regarding the cold start fuel injector in the Fuel Pump Not Priming thread.

I'll address a CIS hot start problem that may be difficult to diagnose. I found this issue with both of my 1979 928s.

The Fuel pressure accumulator is a spring loaded piston in a cylinder that fills with fuel and holds pressure. If there is a pressure loss in the fuel lines the spring loaded piston pushes pressurized fuel into the lines. The two functions of the accumulator are to provide fuel pressure at start-up and to keep pressure in the lines to reduce vapor lock (fuel boiling in the lines when the engine is turned off).

The typical failure mode of the fuel pressure accumulator is the piston seal leaks and fuel drips, as long as the fuel leaks are found (or smelled) the diagnosis is simple. A frozen piston with no leaks is more difficult to diagnose, without the residual pressure hot start problems occur and the most common thought is a fuel pump problem ... however, cold starts are less of a problem thus confusing things.

I replaced the fuel pressure accumulators on both of my 1979 928s. The part numbers today is 928.110.197.31 (old p/ns: 928.110.197.05 and 928.110.197.07).

Those of you who have flown on early Airbus A32X airliners may have heard what has been described as "the barking dog" strange noises prior to pushback. The strange noises are the Power Take-off Unit (PTU) hydraulic pressure accumulator charging. I think the noise has been reduced in newer versions.

Rich
928 OC Membership Chairman
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue soon to be upgraded with "beast" internals
1979 928 Euro 5-liter track beast soon to be cannibalized
2009 Cayenne BaseFuel pressure accumulator.pdf

My 79 CIS 450SL has a lot in common with the 928 and on several occasions I've found solutions to its issues on 928 thread. On the Mercedes, however, they have an fuel "accumulator" right next to the fuel pump under that car and, as best I can tell, it's little more than an empty beer can with in/out fuel fittings. Your description of the 928 accumulators makes me wonder why the Mercedes CIS isn't the same...

[dr bob]

My 79 CIS 450SL has a lot in common with the 928 and on several occasions I've found solutions to its issues on 928 thread. On the Mercedes, however, they have an fuel "accumulator" right next to the fuel pump under that car and, as best I can tell, it's little more than an empty beer can with in/out fuel fittings. Your description of the 928 accumulators makes me wonder why the Mercedes CIS isn't the same...

My memory of the CIS on your SL is that the accumulator function is the same. Bosch was suffering some serious hot-weather issues with vapor lock, and the symptom was common across Mercedes, BMW, Saab & Volvo, plus some others that used rebranded Bosch pieces. The "solution" was to increase the fuel pressure with bigger pumps, and that included stronger springs in the accumulators. The bigger pumps ended up being worse, as they heated the circulating fuel even more. For the 928, there's a small heat exchanger that wraps around the AC suction line between the evaporator/TXV and the compressor. In hot weather, drivers will certainly use the AC, so why not use that cool suction line to soak up some fuel system heat?

Fun stuff!

[Tom]

My 79 CIS 450SL has a lot in common with the 928 and on several occasions I've found solutions to its issues on 928 thread. On the Mercedes, however, they have an fuel "accumulator" right next to the fuel pump under that car and, as best I can tell, it's little more than an empty beer can with in/out fuel fittings. Your description of the 928 accumulators makes me wonder why the Mercedes CIS isn't the same...

My memory of the CIS on your SL is that the accumulator function is the same. Bosch was suffering some serious hot-weather issues with vapor lock, and the symptom was common across Mercedes, BMW, Saab & Volvo, plus some others that used rebranded Bosch pieces. The "solution" was to increase the fuel pressure with bigger pumps, and that included stronger springs in the accumulators. The bigger pumps ended up being worse, as they heated the circulating fuel even more. For the 928, there's a small heat exchanger that wraps around the AC suction line between the evaporator/TXV and the compressor. In hot weather, drivers will certainly use the AC, so why not use that cool suction line to soak up some fuel system heat?

Fun stuff!

Hmmm, that's probably right. I went back and realized the part I remember (empty can) is technically called the damper -- just a reservoir after the pump to wash out pulsations I guess -- even though some call it an accumulator.

Re vapor lock, this car definitely suffered from that. My dad got this car in 1981(ish) just off a 2 year new car lease. My family was set of a summer vacation in the Sierra Nevadas. We teenagers and extended family got there around lunch, but my parents were driving separately in their shiny new 2-seater 450SL. As the afternoon wore on -- no parents. No cell phones back then, and the cabin had no phone. The car had lost all power as dad drove through the 105F Sacramento Valley. They had it towed to a local gas station that had no idea what to do with this liddle fer'n job. To their surprise, after the car cooled off and the afternoon sun went down, the car started right up and made it to the cabin without incident -- a mere 8 hours late. We later learned all about vapor lock.

[dr bob]

Not sure the Merc with CIS has much in the way of pulsations to smooth out. It's continuous injection rather than on-and-off electric valves and a turbine pump, with bypass pressure control at the metering unit draining back to the tank. It might help some with high-load transitions, but not much since the inertia of the fuel in the lines changes almost none with the accumulator next to the pump. But the big deal is maintaining system pressure through the heat-soaking metering unit on hot shut-off.

---

Had a similar issue/adventure in a somewhat similar Sierras location at about the same time your dad's SL was new. Summer heat, 395 north of China Lake, hot-rodded KE-jet Saab 900 Turbo, headed for a fuel stop in Bishop and onward to Mammoth with relatively new GF. Sputtered to a stop. Rumbling from the tank. Loosened the fuel cap and it ejected 20' onto the desert. Hmmm. Fortunately had a bag of ice in the little cooler, spent that on the fuel metering unit. After fifteen minutes of ice plus evaporative cooling from the tank, all was fine. Except that driving with AC off to Independence in 100º+ summer heat wasn't real popular with what's-her-name. A fresh tank of fuel to soak up more heat, and the rest of the trip was event-free.

How spoiled we are these days. Newer cars are relatively bulletproof reliable. The same route in the later 928 was never a worry at all. A few electronic counter-measures in certain places made that commute pretty effortless. Doing it in the Saab was similarly effortless, and not a worry so long as I didn't run the tank level low in summer heat.