GREGGPENN

Ok...gotta run something past our experts.....And...if you are reading this after 3/22/18, skip to page 2 after this OP. It will save you time. (I posted a pic of my plugs from my phone...that wasn't up-to-date).

My build is almost 8yrs old. Seemed pretty solid when new and when I was less knowledgeable. More time and more reading sometimes just leads to more questions but here goes....

I noted minor vibration growing after the first year or so. SpeedTalk thread gave rise to consideration my RA might have a balance issue due to the amount of weight drilled on an external front crank setup. Doesn't seem to be any way of proving/disproving this, plus many have argued otherwise.

It has FELT as much like a mild miss than anything. With self-ported (mega hogged SLP) intake and unique headers, I consider air pulses leading to my perception of vibration. I have checked many of the usual suspects. Can't find a vacuum leak or visible arcing in the dark. Nothing unusual on scans over the years either. Oddly, my compression test showed EXACTLY 150psi on all cylinders which is both low and oddly equal. Could have been a faulty gauge. I haven't tried another because 150psi seems "enough" regardless.

My next hunch are the injectors. During the 3yr interval from start-to-finish on my build, gas sat in the tank. I pumped out most by energizing the fuel pump but didn't drop the tank. So that leads to possibility #3...clogging the injectors over time.

I used a small cam (with AFR 195s) so I've also thought I could get a factory-like idle. I've been close (at times) but I've also consistently felt OCCASIONAL shudders over time. I'm on the 2nd set of front tires and 3rd rears. I've often tried to blame it on tires (since I rarely feel a problem except in low-rpm 6th on the hwy). Shifting gears can eliminate the occasional shudder...which made me think it was that mild miss feeling transferring through the frame.

Last week, it got worse....the miss that is. And, it would definitely cause mild frame shake at the right (wrong?) rpm. That means NOW I have to finally figure out the elusive, long-term miss....that finally got worse. I feel like I need to pull the injectors and send them in to find out how they perform (assuming FIC can do that at various openings). But digging into an L98 is a PITA plus, I'll be tempted to pull the entire intake in lieu of an HSR (sitting in the basement for these 8yrs). Of course, it would require a different dizzy, fuel lines, and minor fabbing. So, it's not quick/easy PLUS I never have time (working on a house rehab). At 60, I'm also kinda slowing down too.

Let me know if you think injectors is my inevitable course of action. FWIW, I also wonder why I've felt vibration in the body so distinctly compared to other cars? I never doubted a spark miss before but I also wonder how much more "tire-like" the effect can be. I've doubted my selection of motor mount -- though never getting feedback on that issue either. Recent frame threads and overall increase in knowledge over the years makes me consider/realize the drivetrain in a C4 is more unique in the way it's mounted and "isolated" from the frame. If I SHOULD feel a miss shake the car, I guess I'll eliminate my choice of motor mount from nagging questions about my build.

Surprise, surprise....I'm a BIG second-guesser! LOL

I can feel the "miss" sitting at an idle -- with my hand on the motor, so I can finally decide it's the engine versus tires/u-joints making me feel it on the hwy. I've had a louder tick-tick -- like a recent vid posted on lifter noise (that was quieted by decreasing lash). It doesn't sound loud enough to be causing a miss...though I suppose it COULD be an injector too/instead? (Seems like it's under the valve cover though.

Way back when, I poked through the injector bung to the intake tract during porting but the forum said "run it". I suppose it's possible 8yr-old injector 0-rings might allow an air leak...through that mistake? (25k on the build).

My spark plug wiring is so unique it took hours to route it BEFORE everything reached final assembly. (Wires go around the back of the engine and up under the headers -- making them "invisible"). I took lots and lots and lots of time making sure none would ever burn/ground out -- though you never know. I suppose one could be arcing though I haven't seen it. Not sure if an electrical meter could SEE/DIAGNOSE that either?

I might take it to a mechanic friend/shop before messing with it much more. I have too many things going on to play footsie with it again this year. He says he has a tool that can measure fuel pressure drop on each injector in an attempt to identify a bad injector. Sounds worth it?

Also, I've replaced plugs last fall. I went one step colder on the plug -- because the electrodes "read" barely lean/hot. Old style A/C plugs. Haven't considered electronics because I installed new coil/dizzy with the build in 2010.

Thoughts on anything else to diagnose?

C409

..... Gregg , Let us see your spark plugs ... if you have a scanner or datalogger , watch how the O2Smv swings like a pendulum constantly from lean to rich and back ... each time the "pendulum" reaches its outer edge (highest vs lowest ... rich vs lean) there could be a slight miss due to the fluctuation of the AFR ... this type of intermittent miss would be almost impossible to pinpoint and might simply be a "that's the way it is" situation ... a HARD miss should be traceable by pulling individual spark plug wires ... which , in invisible wiring situations is likely not easy , OR you can unplug individual injectors while your engine is running to help isolate the mischievous cylinder ... listen to the engine and feel the intensity of the miss vs other cylinders ... you might need to do this several times to get the feel for it ... this won't identify what the exact issue is but it will tell you where to focus your troubleshooting skills .....

GREGGPENN

That's good advice Robert.

If I were to summarize things, I still question choice of motor mounts or if missing should cause mild frame "shaking" like an out-of-balance tire? If that's normal on a C4, I'm fine.

I also think it's odd that the miss seems most apparent a low-rpm loads...and somewhat non-existent at higher rpms. My injectors ohm OK...but maybe one (or more) isn't opening as well in shorter pulse widths?

Idle doesn't seem bad...but hey...it's cammed.

cardo0

Just a few notes/observations that you can consider as you like.

I see no reason for you to use a colder plug range. Your cranking pressure would suggest hotter plugs than stock but you really have to drive and test for yourself.

Your miss reads quite a bit like the wiped lobe my "Target" crate 350 had with the original stock cam. But compression would increase in the affected cylinder as mine proved that - the reduced lobe closes the intake valve sooner increasing compression pressure. But since you don't have that problem you can rule that out.

If you're running headers the exh tube on a miss firing cylinder should be running cooler and you can verify that with a IR gun or a meter that can read thermal couples.

I wouldn't expect a clogged fuel filter or line to be much of a problem at low RPM.

If you suspect a faulty ignition and you don't have data capture electronics a electronic timing light with RPM display you may be able to read a lower RPM on a single cylinder compared to the others just be reading each plug wire at the same tach RPM.

Finally I'd like to ask if you ever considered fast leak down lifters like Rhoads? That might give you more on the low end while performing the same at high RPM.

Hope this can help.

PatternDayTrader

An injector balance test is in order. You probably have one that's partially plugged so it wont really misfire per se. The exact right circumstances have to occur first then you feel the miss. Anyway, that's where I would start.

GREGGPENN

Yeah...an injector balance test is what my mechanic friend suggested as another test (mentioned near the end of my OP). Sounds like you agree? I also like the idea (after almost buying an IR gun at HF last weekend) to measure header primary temps. Renting a different compression tester has also been suggested -- since my readings seemed unreasonably low and, especially....equal.

I found a picture of my plugs last fall...taken a few weeks after swap/install. (below) The set on the left is the new (colder) set. The set on the right has 25-30k on them. As you can see looking at the new set, the tips are still slightly white-ish...like my originals were for many miles. That's why I decided to run a cooler set.

To my naked eye, I didn't notice what LOOKS like a bit of oil on plug #1 (of the new set). They are arranged in physical/corresponding order to the motor -- with the lower right [in both groups] being #1. Could just be the antiseize? Might pull that plug just to make sure.

I have TunerPro. I had TONS of trouble getting connection last fall. I have two ALDL cables and was thinking one might be flaky. I just found the other one this weekend. I could try it again but don't know how to isolate single cylinders using ODB1 parms? EVERYTHING would be easier with ODB2!!!

Seems like I remember reading about Rhoades lifters just AFTER my build in 2010. Sounded like an interesting option (that I might have chosen) but I already have a smaller-than-avg cam in a 383. Seems to have all the torque I can handle. OTOH...If I swapped to an HSR....maybe.

C409

..... ALL of these plugs look pretty rich to me ... rich AFR's will lead to idle stumble . As I mentioned before you can unplug the electrical connector at each injector to see if you can isolate a rough cylinder . Is your TPI MAF controlled ? .....

GREGGPENN

Yeah it's an 89 MAF car. (Hey! I thought it used to say my year under my avatar? Oh well).

Kinda tough to plug/unplug injectors when the intake is hot. Any tips on that?

Never been sure whether to "read" the perimeter or just the tip. Hopefully, you can see the tips look white...at least on the new set. Apparently the picture upload lowers resolution. Here's a closeup of each plug group: OLD...then NEW

GREGGPENN

Geez...I'm trying to remember the sequence of events last summer/fall.

The car was stumbling (dying) after warm-up. I went through all the steps to eliminate the problem AND OF COURSE it turned out to be the final step...at bad O2 sensor. I THINK I replaced the plugs before getting to the correct diagnosis...and even pulled them (for this pic) too. I had a HECK of a time getting the PC to connect to the car. It literally took 70-100 times. But finally, I did get readings.

Back in 2010-2011...I spent lots and lots and lots of time on tuning. Based on BLM values, I was convinced I had it pretty darn close. Oddly, last year I noticed something (originally lost in the fray). I raised my injector constant where it's supposed to be for SVOs but ended up generally lower the MAF tables. Last fall, after measuring FP, I did the calculation and determined my injector constant SHOULD be too high -- to my pressure. When I lowered the pressure (and reset MAF tables to "stock", it ran rich). I thought that was really odd....especially when my fuel pressure measures low (39-40psi IIRC).

I put it back where I'd tuned it...and a QUICK scan looked like BLMs were back where they were supposed to be. Thought about asking the forum(s) on this but didn't get around to it.

The long/short of it is the plug read MIGHT NOT be the best because it might have occurred WHILE my O2 wasn't working correctly. I wasn't driving it long after warm-up because it was dying. So, the plug "read" might be a bit rich since most of their runtime was open loop warm-up...where it's richer anyway.

I can't think of why I might have pulled them AFTER finding/fixing the O2 sensor problem -- so it was probably before.

PatternDayTrader

Plug reading is not what it used to be.
Every one of those looks fine to me. Yeah it might be a simdge on the rich side but that's better than lean.

C409

..... Way too rich ... way too cold ..... Check out this link for what to look for on your plugs .....

http://www.dragstuff.com/techarticle...ead-plugs.html

GREGGPENN

OK...The old ones had 25k on them and were THE A/C Delco plugs recommended by AFR (as shown on their website). The new ones were one step colder (as determined by an online conversion to equivalent Autolite plug...then one step down).

What plug would you try next? If my compression readings were accurate, should I go back to stock plugs? Seems that Autolites have a clearer/better chart/numbering/method for going up/down.

Why do you think the tips are white...if rich AND cold?



I suppose I could also try leaning it out by adjusting the swing point for rich/lean in my BIN. Seems that lower the injector constant wouldn't do anything IMO...because MAF would adjust. IIRC, I switched from the Corvette "swing points" (AYPY) to the ARAP values in order to ensure it wasn't lean. As pointed out, always safer rich than lean. FWIW, I'm not sure mileage would be considered "bad" either. Haven't actually MEASURED recently, but MPG gauge is showing that I'm getting 15-16 avg around town (combined).

383vett

The insulator on those plugs is covered with soot. They are rich. Put new plugs in which are hotter than what you are running now. Some race cars run warmer plugs to get the engine started up to temp. Then colder plugs are used for the race. These plugs look like cold plugs used during a cold startup. They could be the source of your misfire. Do you run a wideband? With unleaded fuel, the insulator should be bright white or slightly discolored. Yours are black. What you are referring to as the tip is the electrode. That should always be clean.

C409

..... Use whatever brand plug you like ... start with the AFR recommended heat range and go from there ... I use NGK's ... their heat range gets colder as the number goes higher ... i.e. , 6 is colder than 5 etc. ... most others are exactly opposite ... higher number is hotter plug ... Check the link I sent to learn how to read plugs for heat range and fuel ratio .....

GREGGPENN

The "OLD" plugs pictured above are exactly that. I assume you are saying I should be even hotter than that? I was looking for a guesstimate like: 1 or 2 steps hotter than AFR's recommendation.

IIRC, The Autolite chart made me realize AFRs starting point was a couple steps below stock (for an 89 at least).

Never did quite figure out plug theory. On one hand better heads (chamber) made me think LESS plug was necessary to achieve good burn. OTOH, bigger cams may have less dynamic compression and need more "heat"?

I assume you refer to link in post #11. I will pull the plugs again and check for evidence of fouling...and (guess I should) go to something hotter than AFR suggests as a starting point.

EDIT: I guess no one is suggesting I try to lean the engine more (vs change plugs)? Taking this assumption one step further, I will assume the compression readings (if correct) point to the NEED for a hotter plug as a "base point"?

Unfortuately, I never invested in a wideband. Back in 2010-2011, I was thinking I had it close based on BLMs and plug reading. Either I had it wrong...or something has changed. Ironically, one of my biggest helpers/advisors/builders back then thought I was a bit on the lean side. Maybe I should see if I can find a pic of the plugs from 2010? Does a new build ITSELF cause a difference in rich/lean hot/cold -- based on the extra heat generated until the engine "breaks in"?

C409

..... Start with the factory recommended heat range plugs then work on the air/fuel ratio until you can accurately read the plugs (without all the soot) ... this might require some heavy footed driving to make the combustion chambers work hard ... then , you can make changes to the heat range if necessary ... its my understanding from reading your posts in other threads , that your cam is on the mild side ... It may take a few "runs" to get some readable color in the plugs .....

GREGGPENN

That's the way I did it back then...PLUS...I need to make sure the sooty plugs weren't a product of the failed O2 sensor I had last fall.

Now that I think about it, descreening my MAF probably accounts for the major changes in stock vs my "ideal" BIN. I probably considered that and didn't ask the forum for that reason.

Looking at my Rich/Lean swingpoints....I started out with everything above 24g/sec at 539 (O2). At some point, I swapped to the automatic BIN where it's "fatter" in the 32-56g/sec range (moderate accel?)

I'll put that back and lower cells (above idle) similarly. I've heard it's better to be fat at idle though...so I'll either leave that the same...or even raise it a hair.

Wonder if should ask.... Is it better to be on the hot side....or cold side with plug selection? For some reason, I always assumed cold.

C409

..... Gregg , ALL of the plugs pictured are sooty ... the old ... the new ... the ceramic insulator should be pretty white on an EFI car ... richer idle air fuel ratios will make your engine idle rough/vibrate ... which , I believe is what you are trying to resolve ...

GREGGPENN

Well yeah....THAT and my propensity to ramble!

C409

..... Really ? ... We hadn't noticed ! ... .....