Hkp7

1984 928S US 4.7
I was driving home from work, still in driveway going around 25mph, when suddenly the engine felt like it lost a cylinder. I have removed each injector plug one by one to check for no change at one of them and all seemed normal. Coil, plugs, wires, cap, rotor, and green wire are all less than a year old. I pulled four of my plugs so far and found no fouling or wear. They did have a yellowish color to them, Bosch Copper plugs. The most recent repair was fuel pump and fuel filter replacement which was just preventative maintainance and that was done around a month ago. I have changed out quite a bit of parts since buying this car a year ago. Its almost to the point of you name it, I changed it. All temp sensors, intake runners, throttle body boot, timing belt, etc. I did consider the possibility that the belt may have jumped a tooth. I removed the passenger side cover due to it being easier, and the mark did line up normally. The mystery to me is why it just suddenly started running like this. It idles smooth, under load it seems like a miss and the exhaust note sounds funny and shakes. I checked all vacuum connections and found nothing loose. I checked cap and rotor for looseness or wear and all seems normal. I guess I will check the remaining plugs and timing mark on drivers side bank. I also want to do a fill up on gas not remembering If I used 93 oct last tank. I drive this car daily and dont believe I have an old fuel issue. My injectors are original and may be some type of failure there as well. Any tips would be appreciated. Bill.

Mrmerlin

swap the fuel pump relay for the horn and change the fuel pump fuse

Hkp7

The fuel pump relay was replaced with new when I did the fuel pump since the old one was a hack job relay cut to fit by po. I will try to swap with horn relay anyway. Fuse, didnt think of that, I have had corroded fuses before. Makes sense. Also if not those items, I did install an ignition switch a few months ago that was from the dealer but had the part number 965 something, I dont remember exactly but I remember reading something about problems with the incorrect switch. I am going out to check the fuse now. Also Mrmerlin, wondering if I could contact you via phone to go over this car, sure would help. My email is Hkp7@live.com. Thanks for the tip. Bill.

thal

there were some recent posts that suggested O2 sensor. you could check voltage output from the O2 to see if it's in spec. another diagnostic i read is that if it is O2 the miss goes away under high to wide open throttle.

Hkp7

I think I might be barking up the wrong tree. Upon tinkering and test driving, I noticed the shaking is worse around turns which makes me believe I may have a transmission issue. Not sure.

Hkp7

I found shaking is worse around turns due to load from power steering, it shakes worse while stopped and turning the wheel. Used flowchart for ignition troubleshooting and found that I have a bad ignition coil. That would explain the power loss under load, the misfiring, the lack of power for months, black smoke when floored etc. I pulled a wire and connected a plug, started the engine, and the spark was weak.

Mrmerlin

I hope the coil fixes the issue

James Bailey

I think the fuel pump relay and horn relay are NOT the same for an 84 USA car...

Hkp7

Correct about the fuel pump relay, it is totally different. Bad news for me on the coil, it didnt do squat. I did all the tests according to the wsm and everything passes except for the lack of power and running rough. I am not sure what to do at this point so I will just keep testing things. Just seems like an ignition problem.

Hkp7

I just checked fuel pressure and it is at 29 lbs. I thought it should be more like 40?

hacker-pschorr

+1
The L-Jet cars have an expensive little relay for the fuel pump with a pin connected to the tach signal that comes from the infamous "green wire".
Later cars with a crank sensor do not need this.

Instead just jumper the fuel pump relay - pins 87 & 30

Is that at idle with the vacuum line going to the distributor? What happens when you disconnect that line?

riviera455

powerloss is deffenitly your ignition,
i had a v8 with msd distributor, i drove to a store only 15 miles from home , this was after a long winterstop. i could make a burnout with the car.
leaving the store , the car was backfireing when starting, i could get it started but it runs poor, less power, but driveble.
a few miles further i stoped at a gasstation and looked under the hood,i thought a lose cable or something, didnt find anything,so,i started again, but it started more difficult and i wanted to drive home as soon as possible ,the car started but runs verry verry crappy and has lost all power, i could only drive 30M/h with gaspedal flat on the floor. i was able to get the car home.
the next day i could not start the car anymore, only a puf and a bang.
cap and rotor where only two years old, had run only about 3000miles. i could not believe those parts are already bad,,but i could not finding anything.
cars that are standing still for longer times in winter get oxidation in cap and rotor, it took more than 3 weeks to get my new cap and rotor because the store shipped it wrong,,
now i always have a new cap and rotorset in spare.
putting the new cap and rotor on and my problem was fixed. car has been in bad weather for 3 weeks and up, just because of a bad cap and rotor. the strange thing is ,,the bad cap and rotor looks both like new, there is realy nothing to see inside,

u can use a strobe light and try to see if all wires give a nice even strobelight at lower and some slightly higher rpm
but the problem mosty occurs under laod, rev the engine freely and it just overcome the bad ignition becouse there is no load,,
dont use sandpaper on cap and rotor, this will only last for short time or ruin them completely.

Hkp7

I jumped out 30 and 87 and pressure read 40. Disconnected distributer vacuum line and no change. I do have the distributer connected to throttle body vacuum and fuel pressure regulators to manifold vacuum.

Hkp7

I noticed that I lost all fuel pressure when disconnecting the fuel relay jumper and suspected the check valve on fuel pump. Then I noticed I had a small fuel leak at the fuel sending unit. For some reason, I thought the fuel was supplied from the bottom of the tank and the line going to sending unit was a return. Then I thought of the earlier cars that had an additional fuel pump where my sending unit is. In that case, the fuel pump gets its fuel from the sending unit. When I removed the fuel pressure test port cap I had an unusual amount of air in the fuel rails and very little fuel. It is possible that I am pulling air from the leak and not enough fuel. Next step for me is to repair leak at sending unit and try again. I will post results. I hope this is the problem.

Hkp7

Now after fixing leak, pressure holds 40lbs for the last half hour. Finishing re assembly and getting ready for test.