engine misfiring at idle only

[Destinmatt]

we got the car I just bought (91 talon tsi awd) running a few hours ago. The fuel pump wasn't getting power in the on position, only start. Long story short we did some fancy wiring and fixed it. When we got it going it sounded like it was misfiring on one of the cylinders, but if you gave it any gas it would go away. It's supposed to be a brand new engine and tubro (big 16g and red top jdm motor). What are some possible causes? I'm probably gonna go get some new spark plugs tomorrow morning just to rule that out. Also the oil pressure gauge is reading zero, but it has oil in it and if that was true the engine would of seized. We had it running for about 5 min before we turned it off.

 

[akdsmer]

A vacuum leak on the intake manifold would cause a misfire at idle and a less noticeable one when the rpms were raised, but I would first verify which cylinder(s) has/have the miss by shutting off each of the injectors (one at a time) either with a scantool/tuning software or the old fashion way of removing an injector one at a time. Removing the spark plug wires doesn't work because each pair of cylinders is a complete circuit, thus removing one wire shuts two sparkplugs down. If there is a weak cylinder, then you will see no change when you remove it. Then you have to determine if its fuel (plugged/bad injector) spark (plug,wires,coil) compression (cam timing off, bad valves, rings ect..)

I had a bad intake gasket on #1 cylinder and it was a bear to find since it was right behind the coil, (I didn't spray to much propane right there !)

And get a mechanical gauge onto the oil system ASAP, (NO oil pressure will make a cheap repair make it REAL expensive (I have one of those in my yard also)!

Good luck and let us know what you find.

 

[ADrewzki]

It sounds as though you may have a large boost leak somewhere. Either that or you somehow managed to mix up your spark plug wires. When I put my engine together, I had left out one of the lower o-rings for the injectors and it made the engine run crappy and made a loud air sound when the engine shut off. Let us know how it turns out.

 

[Destinmatt]

I didn't touch the engine everything is as is when I bought it, and I'm feeling more scamed every second his big list of upgrades is turning into a big list of lies . The engine sounds perfect at higher rpms just bad at idle. My brother says the same thing (mechanic).

 

[Destinmatt]

I should of mentioned that the boost gauge was reading boost, and since the engine didn't sound bad or break after 5 min of running all around the rpm ranges it must have oil pressure. Would you bet it's a bad sensor or a bad guage in the stock gauge cluster. It came with another cluster so I'm thinking about just switching it out to rule that out. Are there any ways to test the oil pressure gauge or the sensors?

 

[steve]

The fuel pump wasn't getting power in the on position, only start. Also the oil pressure gauge is reading zero, but it has oil in it and if that was true the engine would of seized. We had it running for about 5 min before we turned it off.

That's how the fuel pump system works. While cranking the signal from the starts circuit activates the fuel pump relay in the MPI relay. Once the car starts the ECU activates the FP relay to keep the pump running. If it dies after shortly after switching from start to run then the possibles are the ECU not seeing the engine running, the ECU has damage, the wiring to the MPI relay is bad, or the MPI relay itself is bad.

Make sure the oil pressure guage sender is connected. The factory connector is always falling off.

Steve

 

[Xit40]

Definately check for boost leaks that could be a possible cause. My car was doing the same thing a few days ago after I put a new timing belt on. It wanted to stall at idle and seemed to run fine when driving. Check the timing, if it's not spark, wires, or plugs it's definatley your timing. I retimed mine and it ran perfect. unless you get it right on it's not going to run correctly.

 

[DSMcrazy3]

Another possibility could be your EGR system, it could be stuck open. Allowing unneeded gasses to pass though your intake, thus causing misfire.

Just a thought

Josh

 

[Destinmatt]

I knew about the ignition thing we spent 4 hirs going over the car with a chilton book to finally find the source of the problem. there's a bad wire that went to the mfi, and we fixed it. I can't check a whole lot till my brother gets home (not that mechanically inclined), but keep the suggestions comming.

 

[Destinmatt]

I was looking through the chiltons book section on brakes, and it said if your car had to be jumped (you have a dead battery) that it could misfire because of the huge power drain of the abs system. I need a new battery anyway so, I'm gonna hope for the best and try that first.

 

[Destinmatt]

ok, it's a vacuum leak. What are some common places of leaks that would make it mis fire at idle? I was told to spray brake cleaner and if I heard the engine rev and then level out that's where my leak is.

 

[Destinmatt]

ok, it wasn't a vaccum leak. We couldn't find a leak, we changed all the plugs and sprk plug wires. WE checked timing, and we checked all the wiring to the cylinders and injectors. Any other ideas? Might it just be that it had sat for awhile and needs to see the open road for abit?

 

[DSMcrazy3]

Did you try replacing EGR?