Ongoing saga of my rescued STR. Initially had a blown ignition fuse coupled with a bad throttle body. Replaced both of those and now it runs, but with a random misfire code and of course restricted performance. I have done a search on the forums to see what other members have run in to with random misfires without much luck. I have a second STR that runs fine but with a slipping trans, so I am able to swap parts between the two relatively easily. Also the coils and injectors have been recently replaced on this one, and it does not throw a single cylinder code so I think we can rule that out. How would more experience members recommend tracking down a random misfire?

 

injectors could be reman or china garbage, get them tested. don’t count on cylinder specific codes

also monitor fuel pressure

 

I second a problem with an injector. Try running some Sea Foam in the gas and this might help if it is partially clogged. In my case, I had random misfires for a while, used some Sea Foam, problem went a way for a while, then cylinder 8 stopped firing completely. Traced it down to a failed injector.

 

+1 on that. This was my first thought, too.

Also, does your engine have a fitting to connect a mechanical gauge to the injector rail? You want to be sure the pressure reading as seen by the computer is accurate. This would be the same value displayed on a scanner. If inaccurate, the computer will unknowingly adjust the pump output to the wrong value.

 

This is the one that would not start for a while, check the fuel pressure during that work and it checked fine based on the computer readout. Mechaniical gauge indicated it was fine as well. I would think a bad injector would not be a random misfire but woudl show up as a single cylinder?

 

cylinder specific misfire codes can be unreliable. GM didn’t even use them for the longest time

ideally you’d use a misfire monitor but work with what you have

 

OK, did some driving in it today with the code reader connected, kept clearing the misfire so it woudl go. Eventually got it to throw a lean bank 2 code, fuel trims were around +20% on bank two. Then it ran for a bit with no codes.. Got home, pulled the spark plugs to look for a lean cylinder thinking probably had a plugged injector causing one lean cylinder and the rest woudl be rich. Did not find what I was expecting.




Took it back out, the second outing I had fuel trims high on bank one +20% and the random misfires were back.

Checked the oxygen sensor readouts, right around 1.0 lambda. Gave it the beans a bit, same lambda reading.

Mostly interested in actual diagnostic techniques, not throwing parts at it. Anyone have any solid diagnostic tips?

 

What is the fuel pressure with the engine OFF and the ignition switch in the RUN position? What is the fuel pressure at WOT? Please list the readings here.

 

Also, do the usual fuel trim readings!!

Hot engine, parked. At idle then rev to about 2500.

Which of P1000 / P1111 is there?

Which if any unset OBD monitor(s)?

 

Remember that if an injector is not squirting fuel into the cylinder, it will not affect the color of the spark plug. The engine will idle almost normal, but have a slight lope to it. It is under power where you will notice that something isn't quite right. If the injector isn't running properly, you will not see any changes in the O2 sensor readings, but it might pick up a little more O2, If there is a bit of gas still being injected, it will come up as a lean condition/code. I found my bad injector by removing one spark plug coil at a time with the engine running/idling and seeing which cylinder wasn't effected. (With 2 cylinders not firing, it is very apparent.) When you pull of the plug to the cylinder that has the bad injector, you will not notice any change at idle

 

Fuel pressure was in the 370-380 range the entire time, idle, cruise, WOT. Does not throw the code idling any more (it did at first).

If the injector is leaking it will idle rich on a cylinder but run fine at speed usually, maybe a tiny bit rich.

If an injector is plugged it will run a bit lean on the cylinder all the time.

Cannot feel the car missing at cruise or WOT throttle, does have a bit of a lope at idle.

Tijoe, when you did it were you only getting a random misfire code? In the D2 world the ECU is very sensitive to pinpointing the cylinder misfire.

 

Follow what JagV8 suggested. You need to monitor the STFT and then spray carb cleaner around the engine compartment while watching the trims. Are you aware of the very common STR failure of the PB vacuum connection under the SC intake elbow?
Please post back what the STFT's are so you can narrow down what your problem is.
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Monitoring fuel trims under driving conditions you can see bank two in the 20% range and bank 1 in the 10% range. At idle the trims drop to +10 range and +3 ish.

I took the weekend and disassembled the upper intake sections (everything above the SC) and found a couple bolts that were not snugging down all the way (both on bank 2 btw). Replaced the gaskets, repaired/longer bolts and reinstalled everything. Started up and at idle bank 2 is at +12 and bank one bounces around 0, between -2 to +2 on ST.

Unfortunately I dislodged the connector to the throttle motor during the process and since it is brittle like a tortilla chip it cracked and I now will have to order replacement as the throttle no longer works. I will update once I have the connector replaced and can take it for a drive.

 

Ok, test drive with new throttle motor connector installed. No misfire error code if driven normally, or WOT highway run. However mountain run holding car in 2nd or 3rd gear taching the car up from corner to corner results in the misfire returning. I am guessing I either have a marginal injectors or the PO did a less than great job re-assembling the supercharger up after coupling replacement. Now I have to decide if I am keeping it and R&R the SC again with new injectors.

 

I don't see anyway you could assemble a SC to cause the problems your seeing. But if something is leaking you should be able to find it spraying carb cleaner at the connections?
What codes does car currently have?

The fuel pressure should move in a wider range that what you posted? Does your car have a shrader valve? Jaguar dropped this on the later STR's but your 2003 should have one.
It's on the passenger side of the engine at the back end of the fuel rail. Does this spray a good column of gas out when the valve is depressed? We have seen reasonable fuel pressure with a weak pump that still could not deliver enough fuel.
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Also sounds like it could be failing coil(s). But air leaks are more common.

 

Keep at it. Good job so far. When you mountain drive are you near redline or getting close to the limiter. I changed my crank sensor for a similar problem close to redline and it cured it