Any solution for lumpy idle?

[hans moleman]

After a search it seems like a slightly lumpy idle is common with the v8's, I guess I'm no exception. At idle my engine runs well but has a distinctive little "skip" that's fairly consistent, not a big deal but after replacing...all intake gaskets and seals, fuel filter, all 8 injectors, shrouds and seals, ignition wires, cap, rotor, ignition coil, plugs and WUR I had hoped for a smoother running engine. I have found that if I point my timing light at the crank pulley at idle the timing seems to skip around a bit, other posters have mentioned this as well but I have not seen any resolution as to why. Grabbed the distributor and there doesn't seem to be any play in the shaft at all, could it be ignition module, reluctor/p'up?

[calvin streeting]

Mmmm...looks like you have covered most stuff already. How about compresion test? Check the chain stretch? Fuel distrabutor flow test, to make sure all 8 injectors are getting same amount of fuel. Did you say you replaced the injectors, or just the seals?

To others is there an idle stabalition device? But would a faulty one of them cause idle flucuation to juder?

[TJ 450]

It does seem to be quite prevalent, this issue.

In case another lead is interfering with your no. 1 pulse, it would be an idea to separate the lead if you haven't already.

To remove most of the guesswork, I would highly recommend taking it to a shop where they can hook up the diagnostics unit with oscilloscope to measure the waveform... It will tell you whether or not you have an ignition fault. Then you will be armed with the knowledge of whether or not the ignition system is involved.

Mechanically, it could also be a failing harmonic balancer, which is unlikely but possible.

Tim

[koan]

I'd have to disagree with the premise that all V8s have a "slightly lumpy idle", mine is smooth.

Same with timing marks, with variable reluctance distributor and electronic ignition the timing mark doesn't move and is a well defined single mark, not smeared out. Don't know if the type of timing light used makes any difference. For this I'd be checking the distributor's mechanical advance mechanism and lubricating it.

My engine did have an occasional misfire at idle, in warm weather when idle went lower the misfire was worse but if I took the idle up to 850 RPM the misfire disappeared. A compression check showed one cylinder well down and a couple of others slightly down. After a rebuild of cylinder heads including a valve grind and new guides the misfire is gone.

koan

[plmstr92]

Would be worthwhile having your mixture controller setting checked.A lean mix might lend itself to the symptom you describe i believe.

[hans moleman]

Pulling the distributor apart is going to be my next move, the mechanical advance feels smooth when the rotor is advanced manually but checking and lubing is a good idea Koan...however I'm not at all suggesting that "all"our v8's are lumpy just that it appears "common" if you search rough idle or something similar. I'm pleased to hear yours is running smoothly though. A previous compression check yielded a consistent ~180psi for the 8 cylinders, all injectors are new and I have the control pressure deliberately low (about 3 bar) to help negate a lean mixture. The 02 sensor is also new and I'm starting to think it may be trying to compensate for the rich mixture with the frequency valve so I may try to raise control pressure to about 3.2-3.4 bar. If I do disassemble the distributor I will replace the reluctor and pick up while I'm in there as they are not very expensive from  Rock Auto. 

[jbrasile]

Hans,

I agree with you that idle issues are an elusive problem with our V8's and do require quite a bit of fiddling and checking.

Looks like you have covered all the basics. Let me guess, in neutral your idle is pretty smooth but when you select Drive you have a slight shake correct?

A stumble/miss is almost 100% ignition related and can be the result of a bad ignition wire. The 77 6.9 we restored has had absolutely everything replaced in its fuel and ignition systems but it too developed an annoying stumble. At first we replaced the coil and it got cured for a while, however the problem came back and it was one of the new wires that for some reason decided to go bad, replaced it and things improved tremendously. I still think the idle on that car can be improved by tinkering a bit with the control pressure, but first I will try that on my 450SEL which also has the slight shake in D despite everything being new, including the timing chain. I am experimenting with the factory combination of Bosch cap/rotor + Beru wires and Bosch blue coil to see if things smooth out in the 450 and I will post the results here. I can attest to the fact that using a Bremi new cap with the old Bosch rotor did make a difference for the better but that could be because the old cap (about 30000km) did have more pitting than expected.

Here in Brazil we have the issue of 20-30% ethanol in the fuel which does not help. On top of that they  mix in "unknown" solvents and other substances in the gas and that plays havoc with our old fuel injection systems making adjustments quite difficult.

Tks,

Joe

[hans moleman]

That about sums things up Joe. I bought my cap and rotor from a local Autoparts chain so I have never been sure who manufactured them. I have not metered the the cap and rotor but will do so as metering my Champion plugs yielded a little surprise. I also have a little skip in park as well, if you lay your hands on both cam covers you can feel the engine shake a little now and then but what really stinks is that you can feel the entire car shake from inside from time to time. I have been busy with wedding crap this weekend so I haven't been able to do much sleuthing, I'll try to get back in the garage this week.

[jbrasile]

Hans, I tried Champion plugs in the 78 450SEL and they did not do very well, went back to NGK for much better results, you may want to do that too. Funny thing is that in the 1980 450SEL that I keep in Los Angeles, Champions have noticeably improved idle quality, but in the 78 car here in Brazil not only did it become worse, but operating temps went up. NGK's seem to be the way to go on these cars.

Joe

[hi ho]

I've had the same lumpy idle problem from day 1 with my Merc.

Certainly, the ignition system and vacuum set up would be first thing I'd go through.

But, as with my 450, you may find that the lumpiness remains.... I'm almost certain now that the problem is with worn cam lobes.
So I live with it for the time being - but the head/cam change will come

hiho

[jbrasile]

Well, I have just replaced the coil, cap/rotor as well as the condenser wire in the 78 450SEL. The Beru ignition wires I ordered were mismarked from the supplier and had the wrong terminals for the dist cap, so I kept the old Bosch ones which have about 30k kilometers and have checked ok for impedance.

First impressions:

1) Idle speed increased by about 100rpm and has improved itsquality
2) Engine/exhaust note is very clean and smooth

Did the slight shake go away completely, well no, but I was able to leave the idle speed at 800rpm in N and that seems to deliver the smoothest idle. I can live with the results.

The absolutely last thing to check would be the control pressure, maybe I borrow the fuel press gauges from my mechanic over the weekend and check things out.

Tks,

Joe

[hans moleman]

Well in my case the control pressure does not appear to be the source of my crappy idle. I put the CIS gage on the WUR again last night and dialed warm temp pressure in at 3.4 bar, leaning things out a bit. There is a tank of new fuel and the filter has only ~100 miles on it. I put the timing light on it and it is set to 5 degrees BTDC at idle (my low compression slug seems to appreciate the advance). With the light on you can see the timing staggering around a couple degrees. Sitting in the car at a light you really feel the engine shaking the chassis back and forth, frustrating.

[TestJet]

my 1979 6.9 today decided to start running rough at idle. it will run fine at highway speeds but just about quits at idle. it blows a lot of carbin out the exhaust that you can see on the ground. also while sitting at idle i noticed smoke coming out from under the car and the left cadilitic converter was smoking and smelling of something burning. any ideas. does the cadilitic converters ever get stopped up?

[WGB]

They most certainly do get blocked up and they also burn out with rich fuel mixtures.

Bill

[jbrasile]

Idle report:

Maybe I got an exceptionally good tank of gas this time, but the idle on the 450SEL is excellent, clean during the warm up phase and super steady at operating temp, body shake practically gone. As I mentioned before, new cap/rotor, coil and condenser wire replaced about 2 weeks ago, car is running super smooth right now.

Tks,

Joe