Exhaust trouble?

[TommyBatchelor]

My car is currently getting 16.5 MPG. There is an exhaust smell in the cabin while driving. When I have the cruise control set and am going down a hill, the car shakes a lot (kinda like the engine wants to hop out of the car). Also the exhaust makes a popping sound when I let off the gas after accelerating.

 

Not sure if these are all symptoms of the same problem, but does this sound like an exhaust issue? I'm looking at taking it into a mechanic, but want to have a general idea of what I might be looking at first. My Reatta is my only car (which I use to get to work 5-6 days a week) so I don't have the time to work on it much myself right now.

Edited May 6, 2021 by TommyBatchelor (see edit history)

[padgett]

An exhaust leak can screw up the O2 reading which can screw up everything. That needs to be fixed first particularly if the catalyst has rotted out.

[2seater]

If I understand, the engine shakes when not under load, like going downhill?

 

The first thing I would do is watch the O2 sensor cross counts, ED18. It is the easiest way to get an idea if it is functioning properly. More cross counts is better. It needs to be warmed up and driving to get the best overall picture

[TommyBatchelor]

20 hours ago, padgett said:

An exhaust leak can screw up the O2 reading which can screw up everything. That needs to be fixed first particularly if the catalyst has rotted out.

Are you talking about the catalytic converter? Cause the metal cover actually got knocked off a while ago when I went over a large hump in the road. 

[TommyBatchelor]

21 hours ago, 2seater said:

If I understand, the engine shakes when not under load, like going downhill?

 

The first thing I would do is watch the O2 sensor cross counts, ED18. It is the easiest way to get an idea if it is functioning properly. More cross counts is better. It needs to be warmed up and driving to get the best overall picture

Yes the engine shakes when not under load while going downhill. 

 

I'll check cross counts on my way home from work. 

[TommyBatchelor]

Okay, cross counts.

Monitored ED18 on my drive home. When I first started the car and sat in idle it hovered at 45-50. For the first half of the drive it fluctuated around 25-60. 25-35 on level ground. 35-45 on ground with elevation changes. 45-58 while accelerating or coasting down a hill (a bit of vibration while going downhill).

Halfway through the drive it dropped to 0 and stayed there for pretty much the whole rest of my drive. I noticed that the Service Engine Soon light stopped blinking when cross counts dropped to 0, and after I got off the freeway the light would blink again any time cross counts were above zero (seems a little backwards). The vibration also got much worse while going downhill after cross counts went down to zero and the Service Engine Soon light turned off.

Idle at end of drive ED18 stayed at 0.

[2seater]

I have never seen numbers quite like that? This is an '88, correct? I am not as familiar with the touchscreen so I am not certain about a blinking check engine light, but mine blinks when in diagnostic and closed loop. I will need to drive mine and see what mine does again, just for confirmation, but those numbers are extremely high. Ordinarily, that's a good thing, but seems out of range, and the dropping to zero shouldn't happen for extended periods. Almost seems as if dropping out of closed loop for some reason. What is coolant temperature indication?

The previous comments about exhaust leaks and catalyst issues are certainly valid. Coasting downhill the forces on the drivetrain are somewhat reversed, engine vacuum is high and it is holding the car back to a small extent, less so in overdrive above about 50mph where there is almost no engine braking. Does it seem to vibrate more when idling or moving in reverse?

[padgett]

I've never seen numbers like that either, usually teens and 20s driving and single digit at hot idle. I'd check for a loose connection and change out the O2 sensor. What is the O2 voltage reading (ED07) ? (once lit off should be switching from .2-.4 to .6-.8v

[Barney Eaton]

Would a bad cam sensor cause the reading you are getting?    A bad cam sensor will affect performance and gas mileage. 

Note... the sensor is often not bad but the sensing magnet has fallen out of the plastic holder. 

[TexasJohn55]

3 hours ago, Barney Eaton said:

Would a bad cam sensor cause the reading you are getting?    A bad cam sensor will affect performance and gas mileage. 

Note... the sensor is often not bad but the sensing magnet has fallen out of the plastic holder. 

Barney, I drove my '88 98 for months with the light on and missing magnet, great mileage and performance. I replaced the magnet to get the light off and nothing changed. I could not tell any difference. That car would get 27+ mpg on the interstate consistently. I don't know what a "bad" sensor could do, it could be worse than no magnet.

Edited May 20, 2021 by TexasJohn55 (see edit history)

[2seater]

The cam sensor may have a slight advantage in smoothness at low rpms, but unlikely to have any other discernible effect. Lots of engines have been produced without the sensor at all. The little brother of the 3800, the 3300 found in early '90's midsize cars like the Century, operates in batch fire mode and has no cam sensor. Rough figures: At 1000 rpm, the intake valve is only open an appreciable amount for 1/16 of a second every two revolutions, about 62 milliseconds total time. As demand increases from throttle input and rpm, the injector is open far longer than any possible intake valve opening so it is possible to time the fuel shot only at lower speed. 

[TommyBatchelor]

On 5/19/2021 at 4:54 PM, Barney Eaton said:

Would a bad cam sensor cause the reading you are getting?    A bad cam sensor will affect performance and gas mileage. 

Note... the sensor is often not bad but the sensing magnet has fallen out of the plastic holder. 

I've had trouble with the cam sensor/magnet since I bought the car. I payed to have the magnet replaced the hard way (rather than through the sensor hole) and have replaced the sensor at least 5 times. The last time, I asked the mechanic to replace the magnet again but he said it was still there and looked fine. My check engine light is on and I do get the cam sensor error on the crt. I'm guessing there is something wrong with wiring or a fuse somewhere? Not sure if that is worth spending more money on to try to fix.

[2seater]

Only you can decide if it is worth the trouble to look into it further, but if both parts have been replaced, and they are generally reliable, there is something else going on. I do not know where the wiring for the cam sensor is routed on an '88 but if similar to an '89, it runs across the front under the radiator area. I had a cam sensor code on the '89 I purchased and found the wiring cut up front, maybe from being lifted or something but splicing them back together fixed the problem. When the sensor and magnet have been replaced, does the code go away, at least temporarily? Have you cleared the codes to see what returns? Just out of curiosity, if the cam interupter (magnet) was replaced by removing the front cover, did it also get a new timing chain and sprockets?