Fluidplay

Happy Holidays…

Hey guys, I’ve been trying to track down an odd engine shuttering problem for the past week. Here’s what’s happening.

The car starts up perfectly and idles steady at 900rpm. The oil pressure is just below 5bar at start up and drops to and bounces between 3.5-3.75 when warm. The A/F bounces around 14.4-15.1 and vacuum pressure is at just under 16 at idle. I’m using a 3bar FPR with a fuel rail gauge and fuel pressure is at 44# at idle. The car runs normal until I get into a low boost acceleration. Then the engine begins to shutter when the rpm’s come up to around 2500 and idles rough with the A/F running rich and hunting a bit more around 13.9-14.7.

I thought it could be a vacuum leak, but after checking all vac lines, IC pipe, MAF connections etc…all are good and tight. I updated the vacuum system with a venturi delete eliminating the cycling valve, so it can‘t be that. I removed the plugs to inspect and found carbon indicating a rich condition, so I changed them and still have the same situation. The only other thing I can think of is the 02’s. If this is the case, is there a way to determine if an O2 sensor is bad? What else could be causing this to happen???

Any advice, assistance or C4 would be appreciated.

Ballistic-Chapman

PMed you

Tom M'Guinn

Have you checked the throttle position sensor yet? That would be a likely suspect in my book.

Fluidplay

Tom - the throttle positon switch clicked with just a fraction of movement off the stop.
Set up as per LR / tech garage
http://www.lindseyracing.com/LR/Parts/IDLE944T.html

Tom M'Guinn

If they go bad, you can still hear the click, but the switch may not be working anyway. You may want to check it with an multimeter to make sure the switch is opening and closing and that you are getting a clean sweep on the potentiometer. Also, you might try just unplugging it to see if the car revs up better that way -- that would be a strong indicator the TPS is bad. If that all checks out, you might also check the air flow meter with a multimeter to make sure it also has a clean electrical sweep.

Does it rev to 2500 and shutter at that point everytime? If so, TPS is likely. Or do you have to be on boost for it to happen? (If so, check the ignition -- cap/rotor, WIRES, plugs). Does the idle recover after it stumbles for a while?

minho78

Yeahh, that seems to be a symptom of a bad TPS.

Fluidplay

Replaced the TPS last year, although it doesn't mean it couldn't have gone bad. Good thought, I'll meter/test it.
I'm using an LR stage II MAF. How would I measure voltage to it?
As far as when it happens...once it goes in to this mode, it seems to begin shuddering at about 2500 whether I'm in neutral revving the engine or accelerating in gear.
As I mentioned the plugs were definitely fouled (carbon build up) not real heavy, but there. I haven't looked at the rotor and cap.

Did some searching in Clark's - possible causes; DME or fuel filter. Any thoughts?

Tom M'Guinn

If the TPS is a year old, then I'd agree it's probably something else. Was it working fine and then just start acting up out of the blue, or did you recently work on something? What chip and fuel computer are you using with the MAF? Was the MAF working fine at one point, or is this a new install? Could it be just a bad tune? You can see the signal (after any piggyback adjustments) by watching the voltages on pin 7 of the DME.

spyder348

I have been a foreign car tech for many years. If u allready replaced tps its prolly not that. Those maf kits usually come with oiled filters. First clean your maf filaments with gum cutter. Check your ignition components. look inside the cap for corrosion. spray the wires with water. etc. Previously someone said check resistance sweep on the tps because the clicks dont mean a thing they are just for adjustment. Last but no least think about it its not a load issue because idle revving or driving its 2500. Check your power and grounds from battery to starter to engine block etc. also might not be a bad idea to replace your speed and reference sensors.

Fluidplay

Tom - No recent work on it...car has always ran a little rich, but was running fine. It first happened on the way back from dinner with some friends. I started to pass a slower car on the freeway and the car didn't respond like it should an began to shudder/cough. I backed off and cruised for a moment and tried again and it was fine. The following day I drove it around and it seemed fine until I got on it. Then it would go into its' shuddering mode and I had to baby it home. I've since tried it a couple more times and each time the same thing.
I'll keep checking these things over the next couple of days.
Thanks for your input guys.

Tom M'Guinn

Based on that desription, I would take a close look at the ignition--cap, rotor, wires and plugs. Stumbling under load is a classic ignition issue on turbo cars. Don't overlook the wires too--they bake under there and are a common trouble spot. A/f looks rich because gas is not being ignited/burned.

V2Rocket

if you want any help im stuck here till wednesday or so with nothing to do.

Fluidplay

Spencer - If you have the time and can come out to O.C. I would definetly appreciate some help tracking this down.
pm sent

brrgrr

I agree with the previous post on ignition.... I had a similar issue at slightly higher rpm under boost - cut out and shudder - replaced the cap and rotor and all was fixed.

fast951

A few additional things to go over:
- What happens if you accelerate without getting on boost?
- Is the Throttle Plate closing 100% (the spring is working and not jamming)?
- Disconnect the Idle Control Valve, does the RPM drop back to normal?
- Did you pressure test the intake system (even if you replaced the vacuum & hoses, you could have a loose clamp)?