New Motor Idling Rough

snowcat

Registered User
1990 cougar xr7 auto
I installed a rebuilt motor and swapped the tranny for a M5R2. Also changed the exhaust too… stock exhaust manifolds to 2 ¼ pipe to a high flow resonator (I haven’t finished the rest of the exhaust yet) No cats and original O2 sensors in original positions.
When I first fired up the car it started right away and sounded great. After it warmed up a bit I took it for a run. I ran it down the road about ½ mile or so when I noticed my temp on my after market gauge was at 200 and rising. I turned around and headed back. When I got back my temp was 230 and I noticed the fan was not on. I quickly unplugged the fan and ‘hot wired’ the fan to the battery. The fan turned on and the temp returned to about 190. At no time did the stock temp gauge go past ½ way. With the fan ‘hot wired’ I went for another run. I went for about 2-3 miles, car ran great, felt great, temp 195ish. On the way back, as I was down shifting the car almost stalled and when I returned to the driveway it was idling really rough. I shut the car off and checked everything, found nothing. The next day I fired up the car and again it idles rough.

At idle it’s like the computer is searching or a sensor is confused. It almost stalls then revs up a bit, coughs, revs up a bit and sometimes stalls. The exhaust smell is really rich when it does this.
I don’t think there is anything wrong with the motor or timing, when I rev it up to 2000 it smoothes out and sounds ok. (And at first it started right away and idled and ran great) I don’t think any damage was done running up to 230F (I no this is hot but it’s not really hot) I’ve looked for a vac. leak but haven’t found any (yet). Could any damage have been done (to the computer) by unplugging the fan and ‘hot wiring’ it? Also could I have lost an O2 sensor due to the open and unfinished exhaust (no back pressure)? Or have I lost another sensor. I put a code reader on it and show no significant codes. Any thoughts or suggestions would be greatly appreciated.
(The fan issue was a bad sending unit)
 
No, no damage from "rigging the fan"


And my bet is you have a vac leak....What is your vac at idle?

are you running 91+ Octane?

clean the MAF
 
Is the engine basically stock? Other than a vac leak (check the I/C tubes and the big collar nut) the rich condition has me wondering if you didn't fuel foul a spark plug.
 
I'm guessing a vacuum leak too...especially since it happend after a sudden deceleration which spikes the vacuum.

What did you use to seal the IC tubes and the other intake components ?

David
 
I kind of have the same concern. Rough idle in gear, and runs rich. What can cause a car to run rich? I read a post that said that if the fuel pump is going bad it can cause a rich condition?! Has anyone experienced this?
 
He said it was running rich..wouldnt a vac leak cause a lean condition from unmetered air entering the engine?

Corey

When that additional air mixes with what should be the correct amount of fuel (otherwise balanced) and then hits the O2 sensors, they tell the computer that the engine is running lean (unbalanced). The computer responds by supplying more fuel...this is more fuel than is needed, which results in a rich condition in the cylinders...in theory. In actuality, things are much more complicated and situation dependent, but you get the idea as to why a vacuum leak can be a suspect.
 
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(The fan issue was a bad sending unit)

Have you replaced the sending unit yet? It sounds to me like your car is staying in open loop continuosly due to the car not knowing that it is up to temperature. This could lead to fouled plugs/02 sensors as well...
 
I'm wondering if you have a leak at an IC tube connection or the IC itself. The times I've had leaks in these areas I had similar symptoms. When I drove it it ran fine but at idle it was rough and liked to stall.
 
He said it was running rich..wouldnt a vac leak cause a lean condition from unmetered air entering the engine?

Corey

Over rich isn't likely to cause the surging at idle, and smooth running at unloaded higher RPM's. As was mentioned, a vac leak will cause a lean condition... the engine computer says I need more fuel.. I need more fuel...so it gets more fuel..too much fuel and a little less idle air as idle is rising, then the engine starts to die from improper combustion and the engine computer says, I need more idle air... I need more idle air.. oh noo, I need less fuel... I need less fuel. And back and forth it goes causing the smell of a rich condition, and the surging idle.

A vac leak after recent engine re-assembly is pretty common as there are so many pieces that need to fit just right. Especially those I.C. tubes as they need to be lined up just right to get the best seal.
 
With the motor running at idle if possible, it's easiest to locate a vacuum leak by spraying all the connections with carb cleaner. When you hit the spot thats leaking you will notice a sudden acceleration and change in the sound of the motor. If any of the vacuum hoses have blown off the back of the supercharger or the return plenum the same thing will happen. Spray every connection from the throttle body all the way back to where the lower IC tube connects to the return plenum and then spray the connection of the return plenum to the intake manifold.

As mentioned above, most common place for leaks is at the intercooler connections.

David
 
Thanks for the response guys, I won’t get to work on the car till the weekend, but it sounds like I’ll first be checking for a vac leak and my IC tubes.

To answer your questions:
Youg… yes I’m running high octane fuel, didn’t notice my vac at idle but will check and check and clean my MAF
Mike… it is a stock rebuild, I think all cylinders are firing but will pull the plugs and check.
Quickstang… no the sending unit has not be replaced yet.
 
On a new engine install it is common to have air pockets develop within the cooling system. In fact it's not common, its guaranteed. Air pockets will cause hot spots in the cylinder heads and a pocket of air next to the coolant temp sending unit will cause it to read cool when the motor is hot. This will result in no cooling fan and running issues similar to what you have described.

Proper and complete burping of the system seems to be a mystery to a lot of SC owners but the guaranteed solution is to purchase a lever release radiator cap, fill the radiator full, fill your overflow tank to the full hot line, and then run the motor until it reaches operating temperature. Shut the motor off and then release the pressure from the system. The air bubbles will be released into the overflow tank and then as the engine cools the coolant will be drawn back into the motor. Monitor your overflow tank levels and repeat this cycle until the cold coolant level in the overflow tank stabilizes. In order for this to work properly make absolutely SURE that the line going to the overflow tank is clamped on both ends, is not obstructed, and has no leaks. Any leaks in the cooling system will render this process ineffective and your cooling system will not function as designed. If you do in fact have a bad sensor then it should be replaced immediately. Don't know how you verified a bad sensor though.

Driving the car with a new engine without going through the process first AND a functional exhaust system can actually result in permanent engine damage. I hate to say it but what you did was quite foolish. Excessive heat during an engine break in period can easily gall the exhaust valve guides and result in both valvetrain noise and poor valve sealing and may require rebuilding of the heads. Hopefully that did not happen to you but I'm letting you know that it can happen and it has happened.

The most likely simple solution is that you had an air pocket that did not allow the ECT sensor to get a reading and the EEC therefore thinks the engine is cold. The engine will idle much like you described if it thinks it is cold when it is not.

As long as your exhaust doesn't have any leaks it will not affect the computer. The motor neither wants nor needs any back pressure to operate correctly but a leak at the collector will cause faulty O2 sensor readings. It is hard to tell if you have a leak (or any other engine noise!) if you have a very loud exhaust. On a new engine you want to be listening for noises so a functional exhaust is actually quite important.

Regarding 02 sensors themselves, if one of them was damaged from the other engine (blown headgaskets perhaps?) then that will cause the motor to run rich also. Since the 02 sensors are what provides feedback after the motor is running, it is very common for a bad O2 sensor to manifest itself as a motor that starts and runs fine until it has a chance to go closed loop and learn some bad data. After it learns some bad data the only fix is to disconnect the battery for awhile to clear memory and then try again. Disconnecting the 02 sensors completely will cause a whole different set of problems so I don't normally recommend that.

If you managed to foul a spark plug anywhere in this process you will also find that the misfiring cylinder will send unburned air to the 02 sensor which will result in the EEC thinking that bank is lean and as a result it will dump fuel which often times makes the situation worse. I don't know that this is your problem but this can help you tie cause and effect together.

Having said all that it could also be a vacuum leak. IC tubes are a bitch and often times people do not get a good seal first time around. I recommend RTV be used on all IC tube connections. Allow it to set up several hours or overnight before starting the motor for maximum seal strength. Sealing up the blower inlet and outlet is also a common problem for people if they had that apart. A vacuum leak will cause the motor to run lean at idle but most people misdiagnose a lean condition. A lean condition WILL smell, often times like unburned gas because it DOES create unburned gas. When a motor is lean enough to cause a misfire guess what goes out the exhaust - raw fuel and fresh air.

Pulling plugs to check for a fouled one is no fun so if you can get the motor to complete a cylinder balance test that will let you know which one may not be firing. But if the car won't idle long enough to complete the test then that is a another problem.

Good luck, 99.9% says that the solution will be found in one of the things I stated above.
 
To check the ECT sensor I use a ohm meter. Attached is a chart for other temperatures.

When I rebuild an engine I usually replace the sensors. :eek:
 

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found it

Well, I found the problem. It was the supercharger exit tube, the one with the big nut (looks like it didn’t seat properly when I installed it) AND a blown out supercharger intake gasket. I replace all gaskets between the throttle body and the intake manifold. All looked good except the two I mentioned. Did all the IC tubes with RTV. I let everything dry for about 4hrs before firing it up. She idles puuurrrrfect., and the smell is gone. Only let it idle for a couple of minutes, and didn’t rev it up because I want to let the RTV cure longer. But looks like that was the problem.
 
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