Car: Red Thunder II (2004 MCS, 6mt). 166k miles

Issue: sitting at light car sometimes feels like it has a hiccup or stutter, maybe " bloated and gassy" as it feels like something is groaning- like it literally needs to burp. Car is stationary- no load. A/C is on. As you get moving if you are not on it, throttle response is a little laggy, and sometimes there is a slight hiccup or bucking as it gets going. Full tilt, it's fine.

Coolant is topped off. (side note: car was initially fixed up to go to SW FL. I think I may have installed a lower temp t-stat as the car rarely touches the middle heat range- I know that's not ideal, but it doesn't overheat.) Maybe part of the issue? Neither performance, mileage or reliability have suffered because of this.

Not sure if this is the yo-yo issue. I have no clue. Car has upgraded pulleys (15% and 0%) and cooler plugs. Everything seems to be in perfect working order. I know the coil was a stock replacement some 4-5 years ago. Wires "MAY" be original. I do not recall. When car was taken apart 4-5 years ago, injectors were rebuilt. Car starts and runs fine with no issues. Belt tensioner and belt are newer and in excellent condition. This situation is intermittent. Feels like it happens when it's warmer (maybe when A/C is on specifically? Not sure. Haven't really paid much attention)

Thoughts?

 

It could be the AC clutch going out, my AC acts "gassy" in stop & go traffic also. I always turn the system completely off when hitting the highway exit ramp, the vents usually leak out so much cold air that I can make it to where I'm going without dying. It runs just fine at highway speeds though, that's where it's most efficient to operate it anyways.

 

My 03 JCW was doing this for a few weeks then started to overheat while sitting at a light only, changed out the t-stat and fixed.

 

Any misfire (or other) codes? Fuel pressure at the rail good?

 

No codes. I've not checked the FP as it's an intermittent issue that happens once in a while and then goes away. When it happens I tell myself I'm gonna need to do some investigating and then it stops and I forget about it until it comes up again...

 

Symptoms suggest a fuel supply problem. At 166K miles the fuel pump is getting on in miles. But I would not advise you replace the fuel pump just on my opinion. With some cars and a proper OBD2 scan tool you can view/monitor fuel pressure in real time. (Standard OBD2 supports at least one possibly several fuel pressure parameter IDs (PIDs). No guarantees but I suspect you could do this with your MINI. I'd look into doing that to see if abnormal fuel pressure coincides with the engine misbehavior.

 

One easy thing to check even before fuel pressure is the fuel pressure regulator sensor line that comes off the supercharger discharge / intercooler input horn. Naturally, it's tucked underneath the horn where it's hard to reach, or even see. The end is a rubber boot which wears out and comes loose from the barb on the horn's underside. When it does it causes issues. When mine came loose it actually led to a CEL light, followed by an EML light; two things you never want to see together. Found the line, put it back on (used a small flat head screwdriver to help lift it back into place) and two blocks later the CEL and EML lights went out.

 

Didn't think about a loose hose, Those rubber boots are a little flimsy. I'll need to take a look. I have an Autel scanner, but this is an intermittent issue so I'd be hard pressed to have it all set up exactly when the issue started.

Right now I do not suspect the fuel pump although it is old and likely original. I don't have a FPR gauge on the rail -Maybe I'll retrofit one so I can see what's going on with it.

 

Can check out / borrow a handheld fuel pressure gauge at a local parts shop, or pick one up cheap at Harbor Freight, Amazon etc. Screws onto the end of the rail at the Schrader valve. Will need to remove the intercooler to reach it (be careful with the boots; that rubber is often old and frail; might just replace them "while you're in there"), then reinstall the IC before running the car and checking the fuel pressure. An unavoidable pain but fortunately doesn't take long.