Excessive engine braking at constant speed

[aviator22]

Hello all. I am new to the site and am looking for advise from any SHO veteran who has encountered this problem. I will try to be as specific as possible.

Problem: At constant speed between rpm ranges of approxiamtely 2000 to 4000 rpm my car exhibits ecessive engine braking (stumbling sensation)when driving at a constant speed or when adding slight throttle to hold constant speed. The tachometer drop is anywhere from 1000 to about 1800 rpm. The tach and engine for that matter becomes erratic. It is a simular sensation to being in the car with a new driver learning how to drive a car witha manual transmission whom is popping the clutch without letting it out smoothly enough. The only way to stop the stumbling sensationis to either accelerate, downshift and incur an accelaration in lower gear, or let of the accelerator altogether. The problem is reminescent to a situation called full-cut I have experienced in my eagle talon at high RPMs.

My inital guess was a faultered mass air flow sensor or a fuel delivery problem. However, I would think the fuel pump being electric that the fuel flow scenario is unlikely or I would have problem during acceleration too. A throttle position sensor makes sense but I havn't replaced it because when i scanned for codes I got nothing.

To date I have done the following.

1. Checked for loose wiring, dirty connectors
2. Replaced MAF Sensor
3. Cleaned the IAB/ISC
4. Checked spark plugs for arcing
5. Tried Fuel injection cleaner
6. Tried new gas
7. Tried clearing the memory on the ECU

A final note. This problem seemed to start right after replacing the entire AC system. I know there are a few connectors on the back side of the engine that are in the way when trying to get to the evaporator inlet for the liquid line. Coincidence this just started or not?

Any ideas out there?

I have run the codes and I didn't get a single code displayed. I could hear the code scanner running its checks, but it gave no indications afterward. I always though it was supposed to at least display 12 if there were no faults found. Perhaps I need to try again.

<small>[ August 01, 2002, 11:57 PM: Message edited by: aviator22 ]</small>

 

[Mike Kopstain]

Have you run your codes at any time? This seems like it would be an EGR valve or sensor issue.

 

[luigisho]

Aviator try putting the year of your car in your sig. so people can make better guesses. A few differences between atx-mtx and certain years. Have you checked for eec codes? I'm thinking a few things. VSS causes similar symptoms. Possible crank, cam, or throttle position sensor also come to mind. MAF's are expensive and easier to diagnose. If you suspect it is faulty unplug it and it will go to open loop mode and performance should improve. Also IAB wouldn't affect performance at steady or increasing throttle. More toward decel. and at idle. I think code 11 is all clear for GenI's.Just FYI for the next time.

<small>[ August 02, 2002, 12:01 AM: Message edited by: luigisho ]</small>

 

[aviator22]

My apologies. The year is 1989. I have considered the speed sensor as a possibility. I will look into that next. I am avoiding the cam position sensor bacause it seems like a faulty CAS would be causing more constant problems and not just the stumbling at certain times. Thanks for your input. I will be checking back for other ideas As I go.

 

[CharlieSHO95]

In case you haven't tried this, switch the AC off when the car is misbehaving. I just worked on my AC before the convention, and my '89 now sometimes does the same thing you describe. I think it's the compressor cycling, in fact, over the last couple of days I can now hear the belt chirping. I don't think it's low freon as I had the problem right after coming out of the AC shop. I think the problem is a partially plugged orifice tube. When I have time, I'm going to try to find an adjustable valve to get rid of the orifice tube altogether. (anyone know of one that would work?)

 

[sdpatt]

If your compressor is cycling too often in Texas, it would be because the refrigerant pressure is low. The compressor can cause a slight bog, but not drop rpms like you described. The descritpion sounds to me like a secondary ignition fault: plug or wires. What plugs are installed, what are they gapped to and how long have they been in there? What plug wires are installed and is there any oil or water in the spark plug wells? Any problems with these items could cause a misfire that would be transparent to the EEC so no code would be set.

You still need to check the EEC codes though. Here is a great site for the test procedure and the code descriptions.

http://www.dalidesign.com/hbook/eectest.html
http://www.dalidesign.com/hbook/2digit.html

All you need to do is connect a jumper between the SELF-TEST INPUT and SIGNAL RETURN terminals on the EEC TEST plugs near the firewall on the passenger side of the engine compartment. Then just turn on the ignition key without starting the engine. The EEC will run a test of all sensors and relays and report the findings and any codes that were generated while the car has been in operation (not after you cleared the memory). Read the section of the referenced web site to see how the codes are displayed on the CHECK ENGINE light. You will be looking for 2-digit codes.