Trying to resolve my rough idle (I have some symtoms too)

[gniknave]

90 Q45 163k miles JWT ecu & tcu

ProblemCar idles rough, I can hear the exhaust going "putt putt putt" and I feel the roughness mostly under my seat. I feel it most when the car is warm and it's stronger when in park. It's a lot more common on hot days where it's 90-100 degrees outside. I sort of feel a loss in power but not by much.

In addition to it happening mostly on hot days, I also noticed that once in every blue moon after driving for a while, the car won't start back up right away. It will sit there and do nothing. You hear the fuel pump engaging, all the lights come on, there's power, but nothing happens. No click, no crank, nothing.

I've suspected that it might be a bad connection from the FPCU for the starting issue. But I'm not certain. For the idling issue I sort of suspect the IAC (which I'm going to go clean in a minute), or maybe even an injector or two. However I'm also thinking that all of the problems are related somehow as they all started around the same time...when it got hotter outside.

I have already cleaned the MAFCleaned the throttlebody yesterdayRan a can of BG44k through the tankI usually fill up with Shell, but Chevron on occasion...I've NEVER used anything less than premium

Any ideas?

[DAEDALUS]

The roughness could be a few things. Is it missing a cylinder? Not sure what would cause it to sound different. Retarded timing is my best guess. The FPCU won't keep the engine from cranking. Have someone measure the voltage before and while you turn the key. Battery could be going out.

[gniknave]

The car does slightly burn oil too (forgot to mention). But it did this before those problems started. I do notice sort of a harder start than normal when I start the car. Could this have anything to do with the crank angle sensor?

[DAEDALUS]

It could, but it could be other things as well. I would expect a CEL to be lit if the CAS wasn't working right. Unlikely it's working just well enough to convince the ECU it's fine. Would be pretty quick to check the timing--if it's good the CAS is working. I would think you're in the wonderful position of having access to a consult. See what it says about stored codes, coolant temp sensor reading, MAF voltage, etc.

[Q45tech]

The CAS has 2 systems inside with 2 separate connections..........when high speed fails [the 1 degree pulses] ecu limits rpm to ~~2500].

It will crank and run on either [the 90 degree emergency back up pulses].

The usual failure mode is rust dirt on the 360 tiny slots of the high speed mode and at extreme rpms the system loses count and the timing wobbles a bit.......................not firing at exactly 90 degrees apart. High speed misfire only==== thus the rpms drop back to a range where the ecu can THINK successfully by relying on the 90 degree pulses.

Intermintent connections can give running hiccups while ecu switches back and forth between running normal mode and emergency mode.

All the asssumes the physical position adjustment is correct at 15 degrees before TDC................7----10-----15----20---23 will still crank.

[gniknave]

Intermintent connections can give running hiccups while ecu switches back and forth between running normal mode and emergency mode.

This is the reason I was thinking CAS. I can actually give the connector a push and the car will stall when running. Then it'll start right back up like normal. However on my last Q (90 with 220k) when I had this issue there was no going past 2500 rpm like you said. Then again when I went in and fixed it, I found an almost completely detatched wire.

[RobertsnewQ]

The hard start when hot sounds like the classic symptoms of a leaky injector. Does it do it consistently?

[gniknave]

Pretty consistent hard start, but not always...

I finally stopped being lazy and ohmed my injectors. All were right in the 13 range except #6... it was at 36! I repeated the procedure several times to confirm. Looks like this may be the problem. I'll replace it and then check afterwards...

[RobertsnewQ]

Pretty consistent hard start, but not always...

I finally stopped being lazy and ohmed my injectors. All were right in the 13 range except #6... it was at 36! I repeated the procedure several times to confirm. Looks like this may be the problem. I'll replace it and then check afterwards...

A little diagnosis goes a long way!

I'm not up on my injectors - Is that one that can be replaced with the plenum on?

[elwesso]

the answer to that is no..... You "might" be able to do it but your knuckles would hate you!