Engine bogging down with any swift throttle change

[JustJeff]

I love all the input!! Feed me Seymour!! Keep all this knowledge coming!!

Here's what I've found thus far. I change the source for my pressure sensor to the UIM front top port and that did make a significant difference. But I still had a low RPM at idle and if I fluttered the RPM it would still hesitate and bog down.

Throttle cable was part of the problem. It had a lot of slack on it at the back of the TB. I tightened that up and that combined with changing the source for the pressure sensor has made a world of difference. It lops a little bit at idle but idles between 500-900ish idle. I'd raise RPMs up to 2-3k and let them drop back down and idle point would be a little different each time.

It still does a very small hesitation when I first flutter the RPM. Associated with that little hesitation I can hear something out of the ordinary coming from the engine. Sounds like something opening or closing, possibly a vac/pressure source...or maybe the throttle butterflies. It is always associated with the hesitation and only happens if I flutter the throttle. I was working alone so I couldn't stick my head in the engine compartment while someone else fluttered the throttle, but tomorrow I can get a helper.

I did not check timing today because I could not remember which timing marks were which and didn't have a spare main pulley to compare. I had mine powdercoated and then repainted the marks myself. IIRC first one is red/trailing, second one is yellow/leading?

One other thing very strange which may or may not be related to the problems. I have a huge amount of exhaust spray of soot and moisture. It was doing this after the initial rebuild and is still doing it. When I play with the RPMs and get them bouncing up and down, not higher than say 3k I have about a 4-5 spray.



I didn't get a close enough pic, but it's moisture with soot. I tasted the moisture and it's nothing but water...no coolant. Where is the moisture coming from? Is all of that from condensation in the exhaust?

[RETed]

Originally Posted by JustJeff

I didn't get a close enough pic, but it's moisture with soot. I tasted the moisture and it's nothing but water...no coolant. Where is the moisture coming from? Is all of that from condensation in the exhaust?

Wow, you're crazy!
Did you type "tasted"???

Don't worry about it until the engine is properly broken in.
Low compression + blow-by tends to spit all kinds of weird stuff out the exhaust.
Couple this with the car sitting around for a while, stuff gets built up in the exhaust...


-Ted

[JustJeff]

Originally Posted by Rotary Evolution

condensation is normal so i wouldn't exactly say that the spray from the exhaust is a problem but just that the engine isn't warm yet.

standing at the front of the car looking at the engine the mark on the driver side of the pulley is the leading yellow mark, the one to the left is the trailing red mark. the engine MUST be idling below 1k RPMs to set timing otherwise it will advance and not be set properly.

warm up the engine and idle it down to 1k or less(you will hear the engine begin to labor and smooth out when it is in idle mode which turns the engine under base timing until it jumps up off idle), jumper the initial set coupler(not really necessary but it will disable the BAC valve which may hold it above 1k). hook up the timing light to your lead #1 rotor plug, adjust the CAS so that the closest yellow mark lines up with the pointer and lock the CAS. move the pickup to trailing #1 plug wire and it should line up with the far red mark.

to test the TPS, hook a DVOM black lead to battery negative, positive red lead to a small pin jammed into the green wire off the TPS. adjust the screw so that the TPS reads 1 volt with the engine warm and idling at approximately 750-800RPMs.

do a sweep of the TPS with the key on/engine off/engine warm, slowly open the throttle to 1/3 and it should read approximately 4.6v and not have any jumps or drop offs in voltage while doing the slow sweep.

if the idle is being problematic and won't idle down to under 1,000RPMs with the engine fully warm then the thermowax is either stuck open due to an air pocket in the cooling system, is stuck due to age, or the idle air bleed screw needs to be turned in. do this first before anything else, sorry for putting it last.

TPS issues/adjustment and timing are the usual causes for off idle stumbles but it could go deeper to a fuel system problem or a vacuum leak.

if the noise you are hearing when romping on the throttle is a pinging sound, like a ballpeen hammer rapping on a sheet of thick metal rapidly then the timing is too far advanced. your timing pulley on the engine may also not match the hub on your engine if it was ever mixed up before during a rebuild. this is usually distinct in that the engine usually kicks back randomly when trying to start the engine. if this is the case then fix it now or the engine will be short lived as soon as it sees boost.

severe advance knock sounds like you're hammering on the engine with a sledge hammer once and then fades rapidly as soon as engine RPMs come up, when you stomp the pedal off idle.

I've never tested TPS using voltage, I've always done a resistance test. I'll try it your way. Though I didn't understand "with the key on/engine off/engine warm"

A couple things to note, I have a JDM engine so I'm missing a few of the standard USDM parts. I have no thermowax (that is the part sitting on the back side of the TB with 2 coolant ports, correct?) Air bleed screw is on the BAC correct? JDM BACs do not have that screw. What I found yesterday was changing the tension on the throttle cable changed idle RPM. Not sure this is the best methodology, but it worked yesterday.

Pulleys should match as I've only ever worked on S5. So unless a previous owner mucked it up all my pulleys should match.

The noise I hear and bogging/hesitation is when I lightly flutter the throttle. For the most part I can smoothly rise and fall the RPM. It's that swift little start to the RPM going up when it behaves oddly.

Originally Posted by RETed

Wow, you're crazy!
Did you type "tasted"???

-Ted

Haha, I did and I am. Now mind you I did not put a glass under my tailpipe and take a swig. I simply dabbed my finger in the moisture to see if it tasted like anything other than condensation (coolant, oil, gas)