Unstable Air-Fuel Mixture on my 98 GTS

[Joe Dozzo]

My 98 GTS has developed a very strange behavior in the last 5000 or so miles (nearly 4000 were accumulated in the last week on a driving vacation in the Pacific NorthWest).

When cruising at part or constant throttle (and most noticable in 6th), air-fuel mixture cycles back and forth between full rich and full lean (my observation from watching the A/F meter supplied w/S Roe's VEC-1) - about 1.5 - 2 seconds per cycle. Engine tone also varies as the A/F mixture cycles back and forth.

Sometimes it will "go away" by itself after a minute or two and sometimes it takes downshifting and mashing the throttle.

My first thought was oxygen sensor, but I'm not setting any "check engine" codes.

As a test, I took the VEC-1 out of the loop completely (not just setting dials to 0), but with no change or improvement so I don't think the VEC-1's the culprit.

Car has headers and Autolite's with about 8000 miles on the plugs (cooler plug to help combat detonation).

Last, I've been running Super104 Octane Boost every other tankful - again, to try and control detonation. Detonation occurrs only under hot conditions in 4th gear bursts. However, the A/F cycling began before I started using 104.

Anyone have or heard of these symptoms? Aside from a trip to the dealer (car's due for a 20000 mile checkup after my next business trip), any suggestions?

Thanks iin advance!

 

[MES]

Sounds normal to me? Mine cycles all the time unless more than 1/2 throttle or so. When yours stop cycling does it light the really rich light 2nd from the right? if so then that's NOT normal and may indicate a problem with the car. I'm sure Roe can give you a better opinion but these are my observations.

 

[LTHL VPR]

Couple things:
1) Don't worry, your air-fuel readings are normal. It should cycle unless you are WOT (wide open throttle)

2) We have found that 104 Octane boost and most boosters in general, don't really work well and do as advertised and coat O2 sensors and plugs.

3) We have found one product that works GREAT. It is not an Octane booster, but is an unleaded race fuel concentrate. It significantly increases resistance to detonation/pinging and raises Octane levels as well. We usually have it in stock and can sell it in cases. On our tuner's Mustang, he was able to add 20 degrees of timing (over what he was able to do with pump gas with pinging) and still not experience detonation. In comparison, he tried several 'Octane Boosters' adn found no real benefits and could not raise his timing at all without running into detonation. If you'd like some of this concentrate, you can reach me at 408.562.1000.
Thanks!
Hope this helps.
-Wayne (LTHL VPR)

 

[Marc Lublin]

The big question you should be asking is why is the car detonating? This can be extremely harmful for your motor. In my experience with 2 different year vipers, the culprit was a buildup of carbon on the pistons. This created hot spots which caused fuel to ignite at the wrong time. There is a pretty easy remedy if you are interested. If you do choose to drive the car with the problem, make sure you back off the throttle when it starts to happen. When I took my motor out on a car that was detonating I could see alot of wear on the rod bearings from the hammering they were taking.

 

[BigsViper]

Dumb Question: Is detonation like back-firing? What are the symptoms? Thanks

 

[Joe Dozzo]

Thanks all for the comments.

I've had the VEC-1 and A/F meter in the car for almost 10k miles now and MES is right, some cycling is normal as the ECU works to optimize A/F (between 3rd or 4th light and 6th or 7th light). I wish I were better at describing the problem as what I'm now encountering is definitely a new behavior. The "new" cycling is more pronounced and pegs the first and last lights as it cycles.

The detonation (also called pre-ignition or "pinging") only started after I installed headers. It only occurs on full throttle acceleration in a higher gear and Super104 has been very effective for me. With regard to the detonation issue, there are at least 3 other cars (96 - 98 GTS's) here in Colorado that have the same problem - SOME detonation after installation of headers.

I'm certainly open to suggestions on the detonation issue Marc. Is this something you can share online? If not, please e-mail me directly at [email protected].

Again, thanks for your comments and suggestions.

 

[MES]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
The detonation (also called pre-ignition or "pinging") only started after I installed headers. It only occurs on full throttle acceleration in a higher gear and Super104 has been very effective for me. With regard to the detonation issue, there are at least 3 other cars (96 - 98 GTS's) here in Colorado that have the same problem - SOME detonation after installation of headers.
<HR></BLOCKQUOTE>

It may be that you can just hear what sounds like a ping noise with the headers on. Headers have a very thin wall compared to stock manifolds so they are not as quiet. Also a small leak in the exhaust manifold gasket will sound very close to the ping you get with detonation. A light pinging at 1/4 throttle is not really that bad for an engine but at WOT it will hammer your bearings to death. I have no noticeable pinging and use mostly 92-93 octane here in FL. If your using 92+ gas and have pinging under WOT then there is something wrong with your car, carbon build up? There is a trick used on Corvettes which experience the same symptoms I forget the exact procedure. You pore some type of cleaner down into the sparkplug hole, let it sit overnight, then change the oil. Is your car under warranty? if so let Dodge figure it out. good luck.

 

[Marc Lublin]

Joe,
A simple way to remove some of the carbon buildup is to purchase GM's Top Engine Cleaner spray. They make it in spray form and liquid. Warm the car up, remove the 2 tubes leading to the throttle bodies and spray the cleaner in the throttle bodies just above idle. Try to use most of the can and try to let the spray choke the car and make it stall. I have used 2 cans at once, one in each throttle body. Then let the cleaner work overnight. Start the car the following morning, but I would get it out of the garage quickly, because it will smoke like crazy. Run the motor until it doesn't smoke anymore and you are done. I would bet good money this will solve the detonation problem.
These cars tend to run rich at WOT which I guess is why the have a tendency for carbon buildup. I pulled my 2000's motor apart to modify it with only 1000 miles on it. I could not believe the buildup on the pistons. Had to take a razor blade and scrape it off. Made some nice piles of carbon. I had heard of this detonation condition from several people back when I was dealing with the same detonation issue on my 94. I couldn't believe carbon was the culprit, but it was indeed. I would do this soon if I were you, WOT detonation kills your motor!

 

[Sean Roe]

Hi Joe,
Another simple thing you might try is disconnecting the battery to clear / reset the PCM. Sometimes they can get into an odd block in the programming and resetting them helps.