Engine bucking
Thread Starter
Intermediate
Joined: Aug 2015
Posts: 43
Likes: 1
From: San Diego, California
Engine bucking
I have a 85 928 that I bought back in April and have been having some engine issues with. When between 2,500 and 3,000 rpm the car begins to buck and oil pressure starts buncing everywhere when I am lightly on the gas, but when I have the pedal to the floor there is no bucking but there is no power untill 4,000 rpm. I did a dyno run and it shows that 2,750 rpm my HP and torque drop significantly. I have replaced the fuel pump/filter (when I blue threw the filter the gas was black), changed spark plug wires (they were in bad shape), gapped the spark plugs, checked fuel pressure and compression, had my fuel injectors cleaned, replaced both fuel pressure dampers and checked the regulator (diaphragm were blone), replaced all intake seals and gaskets (all were in bad shape and leaking), replaced rotor caps, (which were in bad shape), and I have ignition rotors coming (also in bad shape) and checked vacuum lines (which were completey wired wrong) Doing all of this I have gained power but the bucking is still there. Once the rotors come I am also going to check timing.
Does anyone have any suggestion on what my problem could be? And what should my timimg be at?
The only thing I can come up with is my timing is off or maybe a missfire from the bad rotor? Although the bucking doesnt feel like a missfire.
Does anyone have any suggestion on what my problem could be? And what should my timimg be at?
The only thing I can come up with is my timing is off or maybe a missfire from the bad rotor? Although the bucking doesnt feel like a missfire.
Rennlist Member
Joined: May 2014
Posts: 3,559
Likes: 65
From: South Florida
I had a very severe bucking/shuddering around those RPMs under part throttle...it turned out to be my injectors. When you say you had them cleaned...did you send them to some place like Witchhunter, where they will actually check them out and make sure they are meeting spec?
One other thing that was recommended to me to try was unplugging the O2 sensor and seeing if the problem goes away. It didn't for me, but now I have a fairly faint "wavering" under those conditions, which was masked by the other problem....I'm betting that my be an aging O2 sensor. Haven't tested that theory yet but I did get a new sensor because the damn thing is ancient and they're not expensive.
One other thing that was recommended to me to try was unplugging the O2 sensor and seeing if the problem goes away. It didn't for me, but now I have a fairly faint "wavering" under those conditions, which was masked by the other problem....I'm betting that my be an aging O2 sensor. Haven't tested that theory yet but I did get a new sensor because the damn thing is ancient and they're not expensive.
Instructor
Joined: May 2010
Posts: 224
Likes: 25
From: Auckland, New Zealand
I am chasing my own demon at the moment ( by paying a tech) which has my S4 misfiring at around 2000 revs and dying at idle happens once it is warm. It looks to be pinned down to a faulty crank position sensor ( I hope!) maybe of help to you....?
Richard
Richard
Thread Starter
Intermediate
Joined: Aug 2015
Posts: 43
Likes: 1
From: San Diego, California
Also my car is a 5 speed manual
