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I am experiencing this pre-ignition knocking sound during hard and just right acceleration and at the same time a haunting idle.
I've done research to find some suggestions/inputs for this kind of problem and to list some from the inputs of fellow brick owners like Robert A. Froebel and others as well;
* high temperature of fuel/air mixture in the cylinder
* high compression ratio
* high manifold pressure
* high operating temperature
* low octane fuel (does not apply because I use 93 octane)
* bad knock sensor (does it have to have error codes to replace if it is bad?)
From these kind of topic, there are some that suggests to check or replace:
* intake manifold gasket
* valve job
* knock sensors
I would really appreciate your help and expertise on these matter, thanks again.
chrisq 97 855 T5 70K
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Well, I'm speaking to this issue based on many years of experience building vintage American V-8's, not from direct experience with the 850 motor. You've already hit on some of the primary causes of knock, low octane gas and high compression, high fuel/air mix temp and high engine temp. You've already said you're running 93 octane fuel, so unless the station you're using is pulling something funny you can rule out that cause. Maybe try a different brand of gas just for kicks, you'd be surprised the difference it can sometimes make. I never use Sunoco, had bad performance from it consistantly years ago, never been back. High engine temp, you would see that on your temperature gauge. If the cars not running hot then that's out. High fuel/air temp, unless you changed something in the intake plumbing that shouldn't be a problem. However, if you have a vacuum leak somewhere in the intake route that CAN cause higher running temps, as you've then leaned out the mixture with the additional air thats being sucked in, and a lean engine runs hotter than a rich one. But, the engines ECU should have been able to compensate (to a point) and probably would have thrown a code that would tip you off. Higher compression ratio, unless you put on a milled head, that should be out also. However (theres always a however), large carbon deposits in the combustion chambers can raise the compression ratio if big enough, since they take up some of the available volume within the cylinder bore. Trouble is, this never happens uniformly across all the cylinders, so you would probably notice the car running funny if it ever got that bad. Really though, this is pretty unlikely, you can run up to about 11:1 compression on pump gas (my last Mustang was running 10.8:1 and was fine on Premium), and it would have to be some damn big carbon deposits to get an 850 motor up to 11:1 compression. Manifold pressure, is your car a turbo? maybe there's something going on with the ECU and it's control of the boost pressure? I don't have alot of experience with computer controlled forced induction, so I'm not even going to offer anything here. If it was my car, the first thing I would look for (other than codes) would be a vacuum leak in the intake stream or a bad knock sensor. Also, maybe the distributor has something going on with it. If the knock sensor is saying it's seeing knock, and telling the distributor to retard the ignition timing, maybe the distributor is not getting the message, or not able to do it. Still, I would think all these things would throw codes. Either that or you've got some really bad gas...
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hellboy,
Thanks for the fast reply. I'll answer as much I can to your post.
I only use Exxon or Mobil to run the car and recently change the fuel regulator but did not solve the idle.
Engine temperature remain normal on the gauge, the hand is little over the midline w/c I'd been seeing since then. I've also replace the ECT sensor, thought that it's not sending the right signal to the ECU but again, no success.
I've also cleaned the TB w/c showing not much of a deposit including the rear of the AIR HOSE w/c has an amount of oil and cleaned it thoroughly, again no success but notice a high rev during cold start.
Up to this point I'm still wondering if the knock sensor is bad since its not throwing any codes. I remember that I pulled codes P1310 and P0300 last January pointing to Ignition coil 3 primary feedback circuit and the other code as multiple cylinder misfire. The steps I've done was to change the upgraded NGK plugs back to Genuine turbo plugs and reset the codes and did not return since then. At the time, I'm burning oil as well and using 10w30 synthetic.
I've also heard about the use BG-44K to remove deposits if there are to improve compression and been planning to use before I change my oil in 2 months time.
And yes, I'll be replacing the intake manifold gasket and knock sensors if time permits. I already have the parts but just waiting to have spare free time.
You did not mention anything about valve or ring problems on your post, I guess it does rule anything like that?
Hope I've described and clarify your concerns on my problems. Thanks again.
Regards,
chrisq 97 855 Turbo 70K
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OK, you've said that you had codes some time ago concerning ignition coil feedback circuit and multiple cylinder misfire. I find it wierd that a plug changed alone fixed that, but whatever. I would have thought, with those codes, you would have had a distributor or coil problem, something common to all cylinders. I've seen bad coils do all sorts of wierd things to cars, including screw up the radio (go figure). I don't know if a bad coil could cause knock though.
As to rings and valves, you've said you are already burning oil. There's only two possible causes for that, bad valves (specifically valve guides and/or stem seals) or blown rings. How many miles are on the car? If it's less than 150K I'd say its pretty unusual to see bad valves or rings in an 850 with only that much mileage. As problem prone as these cars are, the main engine internals do seem to be pretty bulletproof. You never see posts of people with thrown rods, spun bearings, or the like. I would say the oil burning is probably due to bad valves before I would suspect rings. The odds are higher valve guides and stem seals will go before rings. However, I don't think either of these would cause knock. If anything, they would do the opposite, since bad rings and valves will eventually cause a drop in compression, not raise it. I guess if the oil burning was bad enough you could have the entire face of the piston and the bottom of the head covered with varnish and baked oil residue, and maybe that could cause some knock, I don't know.
I'm not familiar with the product you mentioned for removing combustion chamber deposits, so I can't say if it has an effect or not. The old school technique for removing cabon buildup in carburated engines is to pour water down the carb throat very slowly and let it vaporize in the intake stream, then take the car out and give it an Italian tuneup. Water breaks up carbon deposits very well. My 1973 Triumph TR6 manual actually lists cylinder head removal for carbon deposit cleaning as part of the 60K mile service. Anyway, this doesn't apply to a modern car though, I'm just rambling. Sorry I couldn't give you any better answers.
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posted by
someone claiming to be VOBD
on
Wed Aug 11 09:56 CST 2004 [ RELATED]
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This may seem obvious but when I thought I had this problem with my '96 855T5 it turned out that the throttle cable was banging on the bulkhead (firewall) under heavy load conditions. I just wrapped it in some plastic tape. Worth a try?
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posted by
someone claiming to be chrisq
on
Wed Aug 11 13:07 CST 2004 [ RELATED]
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vobd,
Thanks for the input, it's worth a try looking for this fix.
regards,
chrisq 97 855 T5 70K
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