Hey All! Hope everyone had a happy 4th! I have a '99 Coupe with a weird problem that hopefully someone else has had the pleasure dealing with.

The car:
'99 coupe with 76,000 miles on it. Only mods are a CAI and stainless exhaust that were installed before I got the car. Before this, its been running great.

The problem:
In the past couple months, my '99 started feeling like it was trying to stall if I came to a quick stop on the road down from >40mph. I was scared it was something in the tranny acting up (i.e., the TC clutch solenoid) but I was able to rule that out by putting the car in neutral while I braked.

Anywho, now it does it consistently. With the car in the garage, if I just start it and lightly depress the gas pedal, the RPM's pickup to about 700-800 and then stay in that range. As the pedal is pressed further, the RPM's bog down into the 600 RPM range. It seems like after I press the pedal to the point that the RPM's would be 1000-1100, the RPM's jump up to 1000-1100, and its fine from there. On the highway, same thing happens.... car bogs a little until I get the pedal down. If i'm at speed and stomp the pedal, no hesitation at all. What really sucks, I have never gotten a DTC code!

What I have done:
As I said, ruled out the transmission. Fuel pressure in the manifold is a rock steady 59-60 psi. I swapped plug wires just for curiosity sake... no change.

The good news is, I have a '98 Coupe sitting right next to it in the garage so troubleshooting parts becomes a bit easier.

If anyone has any input at all, I'd really appreciate it. This has been stumping me for quite a while now. Thanks much!

 

Check the intake plumbing, any air getting in post MAF can cause strange idle behavior. Clamps tight? Any cracked rubber? Might want to clean the MAF and throttle body as well.

 

I knew I forgot to add something! I did previously clean the MAF sensor, throttle body, and air cleaner. I will check all the plumbing as well to be sure. Thanks for the response!

 

You, I agree.

Bill R.

 

I gave it a quick look over on the way out the door to the airport for a quick business trip. I didn't see anything obvious yet. When I get back, I'm planning to give it a very detailed inspection. I have read posts about people using cigar smoke to chase out a leak..... any validity to this method?

 

Oh man, yes there is. Firstly, there are smoke test machines that shops carry that plug your system up and pump smoke through. Leaks found that way are pretty easy, just find the smoke.


Cigars could work, not sure about producing enough smoke for the whole intake system. I have used a cherry flavored cigar to check for a boost leak on my Mazdaspeed3's front mount intercooler system before. It worked! But it tasted like crap and by the time we found the leak, we were both properly messed up. The crap you try when you're 17...

 

OK so I finally had some time to get back in the garage today. I gave the engine a good visual inspection. Didn't see anything obvious in the vacuum leak department. I started it up and let it get into closed loop.... then sprayed some WD-40 around everything going into the manifold... PCV plumbing, brake booster line (at the booster and manifold), all the air bridge clamps and ports, and throttle body mating seams. If there was an RPM rise, I didn't hear it!

The only thing I couldn't directly spray is the HVAC line off the back of the intake manifold.... and for that matter, the round "plug" that the MAP sensor mounts on. For what its worth, the HVAC controls seem to respond pretty normally and have no effect on the engine idle. Is that round piece even removable? I was trying to use the "remove the manifold" as a last resort, but if it needs to happen, so be it...

 

Your PCV pluming on the back of the engine behind the intake has been well known to crack and leak. Major air leak. Inspect ALl of the PCV tubing, and rubber bits.

What are your O2 sensors doing??? Can you get a scanner an monitor their outputs? They could be bad and that will effect your idle in closed loop.

I always use some CARB CLEANER when I do my leak test. It shows up better/quicker with pronounced idle changes. I've also seen people use PROPANE or MAPP Gas to check for leaks and that seems to work well also.

If the engine idles well in open loop, and idles crappy in closed loop, look towards the O2 sensors not function properly.

BC

Bill

 

I've not used carb cleaner before, but I think I have some starting fluid/ ether in the cabinet that I can try!

When I looked at it last week, I had removed the PCV tube and the little 2-to-1 splitter. The splitter looked aged of course, but didn't reveal a leak with the WD-40.... maybe the ether will give better results.

Also, let me clarify... and this is my fault for titling the thread the way I did. The car stumbles the most slightly ABOVE idle... in the 700-900 RPM range. At plain idle, it does hunt a little bit, but not like it does in that range. Apologies for the confusion.

With respect to the O2 sensors, what should I be looking for as far as an indication of a bad sensor? I have a live data scanner... just need to know what I'm looking at. Thanks!