RPM Surge and Slightly Rough Idle

duner

Registered
I have been experiencing a problem with my new Mark. When the engine is warm and idling there is a slight surge in RPM. Maybe 50 to 75 RPM. This is with the AC and lights off (doesn’t seem to make a difference with these things on). When driving the car the same surge occurs. What’s interesting is that the surge happens at different RPM. It doesn’t matter if I’m at 60 in OD using cruise control or at a steady 3500 RPM in second gear. The surge feels worse as the revs increase. Another interesting thing, the problem gets progressively worse the more time passes after disconnecting the negative battery cable for 30 minutes.

I have been searching several sites and have taken several steps to fix this problem. First there are no codes (bought a cheap code reader). I have changed the fuel filter, spark plug wires and spark plugs. I should also mention I had a bad driver’s side O2 sensor that was replaced. I found a few receipts from the last owner. It appears the he replaced the fuel pump about nine months ago.

I would have replaced these parts anyway. I don’t want to keep dropping dough without a good chance of fixing this. The first thing I did to the Mark was the HID conversion. So I need to be smart with the parts cash.

Any help would be appreciated. I’ve read tons of posts about rough idle, coils, plug wires etc… But I couldn’t find a post describing a similar problem.

Thanks,
Keith
 
RE: RPM Surge and Slightly Rough Idle

My idle problems, which mainly were high idle (1200) after stopping, happened when it was over 60 degrees outside. The problem got progressively worse over two year, generally during summer time. The fix was changing the idle air control valve. On a Gen1, you will never do anything harder than that.
 
RE: RPM Surge and Slightly Rough Idle

Believe it or not...

It sounds like your MAF may simply need cleaning. This problem has been addressed in a TSB from Ford. Since you say disconnecting the battery improves the symptoms, IMO you are resetting to base values and over time the PCM learns new values from a malfunctioning sensor. The MAF can be dirty and is one sensory input not prone to throw a code.
 
RE: RPM Surge and Slightly Rough Idle

Thanks for the responses guys. Sorry it took so long to get back to this topic.

First, no MODs yet. I was running a K&N filter. That caused problems. I replaced the K&N with a good paper filter and carefully cleaned the MAF with electrical contact cleaner. This cleared the problem caused by the K&N.

I have posted this question elsewhere and it was recommended to change the PVC and EGR and check for vacuum leaks.

Several things have happened since I started this thread. I have changed the plugs and wires. This helped a great deal but did not completely solve the idle problem. I found post concerning a similar problem caused by the AC system cycling. I have been able to almost clear the idle problem and have determined the cycling of the AC system is the culprit when cruising. Troubleshooting multiple problems is always fun.

How noticeable is the cycling of the AC system while cruising in your Mark VIII?

I’m not experiencing a huge surge/stumble; it’s slight but annoying.

Keith
 
RE: RPM Surge and Slightly Rough Idle

Another thing you might check (while you're at it!) is your belt tensioner and pulley. When these start to wear, odd vibrations and changes in RPM have been known to happen...

Just a thought :)
 
RE: RPM Surge and Slightly Rough Idle

Thanks for tossing that in Scott. I'm developing a similar problem. It's about time for a new belt on mine. Is there a grease nipple or some lube point on that belt tensioner?
 
RE: RPM Surge and Slightly Rough Idle

Barry, I don't mean to butt in, but get a Goodyear Gator belt, or the PolyCog by Dayco. Both have much better tensile properties and diagonal grooving on the vee side to combat typical belt noise. I have seen post declaring these belts to be silent. I started hearing a slight noise from mine(or so I thought after hearing of the Goodyear Gator belt) and bought the Dayco PolyCog. I'm very pleased to say the belt did make a difference.

The tensioner is 'lifetime lubricated' as far as I know. If the belt does not silence it, check the tensioner and/or the idler.
 
RE: RPM Surge and Slightly Rough Idle

Please JP. ALWAYS 'butt in'. Your advice is like the Holy Grail of Mark VIIIism.

The problem with those 'lifetime lubrications' is that the manufacturer always has a different perspective of time than I. :)

BTW Driller, if we ever end up at the same bar, you'll never have to buy your own beer. Thanks again.
 
RE: RPM Surge and Slightly Rough Idle

I have posted this question elsewhere and it was recommended to change the PVC and EGR and check for vacuum leaks.

Check for any air leakage whatsoever between the MAF and throttle body. Pay particular attention to the rubber gasket at the throttle body, the PCV hose, the 1-1/2" hose connection at the intake resonator and airbox hose connections. Is the MAF stock? I'm assuming it is because you said NO MODS, and you replaced the K&N filter with a paper filter so I guess that means you have the OEM airbox with panel filter. If the MAF is not stock, you may have an airflow anomaly across the MAF sensor. This may be minimized by rotating the MAF so that it is in a different position relative to the airflow. However, I do not believe that would be the case with a stock intake setup.
 
RE: RPM Surge and Slightly Rough Idle

Hmmm. When I removed that upper resonator I put a plastic cap on the airbox port but it's not exactly SEALED TIGHT, I don't think. Guess I oughta' put some rubber cement on that puppy.
 
RE: RPM Surge and Slightly Rough Idle

Yup. Getting a tighter seal on the intake resonator port seemingly solved my problem. Driller does it again.
 
RE: RPM Surge and Slightly Rough Idle

Check for any air leakage whatsoever between the MAF and throttle body. Pay particular attention to the rubber gasket at the throttle body, the PCV hose, the 1-1/2" hose connection at the intake resonator and airbox hose connections. Is the MAF stock? I'm assuming it is because you said NO MODS, and you replaced the K&N filter with a paper filter so I guess that means you have the OEM airbox with panel filter. If the MAF is not stock, you may have an airflow anomaly across the MAF sensor. This may be minimized by rotating the MAF so that it is in a different position relative to the airflow. However, I do not believe that would be the case with a stock intake setup.

driller,

The MAF and airbox are stock. I did clean the MAF carefully and thoroughly. Can there be any permanent damage caused by filter oil? I just changed the PVC and didn’t notice a change or any leaks. I will spend some time looking for leaks and report the results.

I have solved one of the multiple problems I was chasing. The car cruises very smoothly (with AC system off) after the corrections/maintenance I’ve done. I’m still chasing the “Slightly Rough Idle” and the slight stumble caused by the AC system cycling while cruising.

Please answer the question about the AC system cycling. Now with the car cruising smoothly I definitely notice the AC system cycling. This is not a huge stumble but it’s noticeable, maybe 50 to 100 RPMs. I don’t want to get too far off the topic. The AC system cycling stumble seemed to be masked by uneven RPMs while cruising before I changed the plugs and plug wires.

I’m making progress and appreciate all of the responses.

Keith
 
RE: RPM Surge and Slightly Rough Idle

I do not know of any cases of permanent MAF damage from filter oil such as used on K&N filters. The MAF can get dirty without oil, but obviously are at more risk with oil contamination. The wires used in the MAF are basically similar to incandescent lamp filaments - very fine and fragile. So it has to be said, any physical contact, however careful, can put the MAF at risk.

The MAF can be damaged from silicone products! Silicone can be from sealants, lubricants and cleaners.

I think you are right that this is more than likely a multiple problem, but I feel the stumble with the AC cycling is simply an amplification of a air/fuel problem. If the MAF is damaged or dirty, a lean condition will develop. If you are leaking air beyong the MAF, your A/F mixture will be lean. In the old days, if you adjusted the carburetor too lean, it would miss or 'stumble' at acceleration or a sudden load. You would simply open the jet ever so slightly just to the point where the 'stumble' would be gone.

BTW, after cleaning the MAF, did you clear the PCM by disconnecting the battery for 30 minutes or more? Also, have you had the system scanned for codes?
 
RE: RPM Surge and Slightly Rough Idle

I didn't think the MAF was permanently damaged from the oil but I thought I'd ask. I cleaned the MAF with electrical contact cleaner and didn't touch it.

I purchased a code scanner from Autozone. There was a code that indicated a bad O2 sensor. The drivers side sensor was replaced. This cleared the code. The passenger side hasn't been replaced yet. I just scanned for codes. There's only one. Code 634 - Electronic shift transmission Manual Lever Position (MLP) sensor out of range in PARK.

During this process I have cleared the PCM several times. Right after changing the plugs and wires and clearing the PCM I took the car for a test drive. The car was not as smooth as I expected. As time passed and the PCM has relearned, these symptoms have lessened. That was 3 weeks ago. Things have stabilized.

Thanks again for the pointers. I’ll spend some quality time this weekend looking for air leaks.

Keith
 
RE: RPM Surge and Slightly Rough Idle

Just a note on that cleaning of the MAF sensor. It may smooth out your shifting or shifting issues. See the following except from the manual concerning MAF sensor:

The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulse width in order to provide the desired air/fuel ratio (Figure 37). This input also used in determining transmission electronic pressure control (EPC), shift and torque converter clutch scheduling.



Amazing, it all comes full circle at times... round and round we go ... what happened to the old days of adjusting the vacume flow assyb where the vacume line came in ..LOL

I know I know .. its all relative. :) Just someother FYI
 
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