I have a 98 SL2 automatic with 103,000 miles. When traveling at highway speeds, I have a backfire in the exhaust when I take my foot off the gas and decelerate. When I take my foot off the gas it will backfire 3 or 4 times and then run like normal. It does not backfire when I accelerate, or in park when I rev the engine. I will also notice the backfire when coasting down a steep hill when my foot is off the gas.

I have changed the plugs and wires. The resistance of both coil packs are 7700 ohms, I believe the book says the should be between 8000 and 12000 ohms. I have found no apparent vacuum leaks.

I would appreciate any help on this. Thanks.

 

Actually its called afterfiring since it occurs on the exhaust side and not in the intake side which would be a backfiring problem. One possible reason for the afterfiring could be excess unburned gas allowed into the exhaust systembut where is anybody's guess. A leaking fuel pressure regulator allowing fuel through the vacuum line?


The idle should be around 800 rpm.

 

the idle is close to 900 rpm at normal operating temp.

I believe the pressure regulator is built into the fuel filter on the 98 if i'm not mistaken. I did a fuel system pressure check and the pressure at idle and the pressure decay were both well within spec.

 

Thanks, I was wondering about the idle speed and since you posted it I'm wondering if the idle air control valve (straddling the throttle body adjacent to the throttle position sensor) is a little sticky. The throttle body may need some cleaning in general, the iacv cleaned with electrical contact cleaner because of the precision servo motors driving the valve opening and closing.There is always a possibility of the engine coolant temperature sensor (ECTS, original resin tipped one) cracking thatsends incorrectsignal data telling the PCM that the engine is always cold and keeping the mixture rich that tends to have the engine running a higher idlebut you said the engine is idling near 900 rpm. Typical cold starting/running is witha 1200 rpm high idle continuously lowering ascoolant temps rise until 10minutes later while driving when the engine is warmed up and the idle returns to 800-900 rpm. A bad coolant sensor won't allow this type of engine run at all, either high idling all the time or very lean, stalling out unless pedal effort is used to keep the engine running until fully warmed up. The only way to tell is to remove the coolant sensor and examining it for cracks or if it is still the original one and replacing it with the brass one for reliability. The two-wire connector corrodes from the leakage of coolant and would require replacing, either from a junk yard air temperature sensor connector, pc connector, or dealer's $30 part. Everyone having coolant sensor issues have never described any afterfiring so I may be barking up the wrong tree but you have the information anyway.Edited by: ducky

 

Thanks for the information. I have replaced the ECTS with a brass tipped one, and i replaced to connector as well.

The Throttle body appears to be fairly clean, I cleaned it maybe 6000 miles ago, but i'll have to remove it and check the IACV also to be sure.

 

I checked the throttle body and IACV and both seem to be clean and working fine.

However while inspecting the exhaust I found a hole just before the muffler after the cat. Do you think that a hole in this location would cause a loss in back pressure and possibly cause the after fire.

 

Anything is possible but I wouldn't think that a small hole would create the afterfire condition because the engine is always running and the backpressure is just relieved a little. On second thought if the throttle is closed during deceleration there is a distinct possibility that air can be sucked into the exhaust upsetting the flow and the extra air creating the popping sounds.I certainly would try repairing the hole temporarily if possible to see if this is the cause of the afterfiring or replace that part of the exhaust system otherwise if you intend to keep the car invest in the replacements, perhaps online as there are more places to shop and sometimes better prices. You can always go local and check the shops for best prices for quicker repairs at reasonable cost. Here in the Big Apple (NYC)we have to pay through the nose. Of course I could use my brother-in-laws collision shop for most repairs and sometimes I do. I need to take advantage of family, I'm so lame sometimes.

 

Issues on decel....


1) Ever cleaned the EGR valve?


Pull, inspect for free movement, and clean with carb cleaner (or replace if it is too far gone).


I don't know that I've ever heard of an afterfire/backfire (aside: can you really tell which it is while traveling down the road...) issue traced back to an EGR problem but it is worth a try


2) If EGR doesn't help, I might go for replacement of DIS (actually, bite the bullet and get the entire assembly (2 coil packs + DIS) since your Coil packs are on the edge....


Logic is that the spark advance/retard (timing) info is in part influenced by the speed of the vehicle and whether you are accelerating / decelerating; if the DIS is having an issue, it may be routing spark at improper times....I would think something this nasty would throw a code, but........


Derf

 

sorry for the late update.

I finally was able to replace the exhaust pipe and the muffler as well. This has solved my problem, the car in running like new. Thanks for the help.