Problem with Intermittent Misfire, Worsens at WOT

#1

So at VIR in late March, I ran my first event with my newly refreshed engine. At first it went like stink. Until it didn’t. I had a used engine in after I lost this one last summer, and refreshed lots of parts with the used engine (cap, rotor, intake boot, hoses as required). So the first session was good, but power cut out hard entering hog pen (downhill right hander) one time. Second session had the same result (not sure where it occurred but it cut out again) but still ran great otherwise. Third session I noticed a rough idle in pit lane, which turned into a dead miss at WOT going on track. Would occasionally clear out at ~5500 RPM+ but generally not. After cooling off awhile, it was slightly better (slightly less than continuous miss at WOT) on the out-lap, but then the same symptoms resumed within 1 lap. Trackside troubleshooting had us suspecting a fuel injector because one plug (#2) was wet and black while the others showed lean. Changing plug and testing, then ignition wires and testing had no effect. Sometimes it would clear up but always returned. Loaded on trailer the next day and it missed terribly (possibly more than 1 cylinder miss) such that it stalled at near WOT pulling on the trailer. Strongly suspected the #2 injector at this point.

At home, rolled it off the trailer and into the shop without starting. All plugs showed black, so single injector less likely. Tested everything on ECU harness connector and everything checked out per Bentley manual specs. Coil resistance measured out of spec (7.39k ohms instead of 5k+or-1k spec). ECU temp sensor was slightly (<10%) out of the allowed range at 70F per the Bentley manual. Checked injector resistance, all checked out. Replaced injectors, assuming that one had been plugged. No change. Checked fuel pressure, and it checked out (~43.5 psi with engine off, manually running fuel pump and ~40 psi at idle). Changed coil with one I had on hand that came from my eta car (same part number) that tested at 5.25k ohms (well within spec). Thought I had it fixed. Pulled onto the trailer fine, took it to the dyno a week later. Oh, during this test process, I pulled the AFM and a used spare I had, and tested it by applying voltage across the terminals and measuring a linearly increasing voltage output with flap angle and no dropouts on both units.

Ran pretty good on the dyno until I did the first actual pull, where it hit 100 hp before starting to miss hard and power dropped to <80 hp, and RPMS wouldn’t climb steadily (thanks to the miss). Measured fuel pressure and observed drops below 40 but this was on overrun, not under load. We didn’t have a wideband O2 sensor in the exhaust for that pull, but afterward it would run really lean while missing. Unfortunately no data on when it was running correctly (however briefly). Suspected fuel pump. Changed fuel pump. No change. Changed fuel pressure regulator. No change, but fuel pressure measured slightly lower and took a little longer to settle back to ~40 psi at idle with new FPR. Changed fuel filter, though it was new prior to the March event. No change.

Thought about the problem and remeasured the crank position sensor. Measured just within spec (540+or-10%, measured 486). My new Hella branded spare measured 471-48x. Changed it and found no difference. Tried my chipped ECU (which I routinely run at hill climbs, so I know it works well). No change. Swapped fuel relay from my eta car. No change. Swapped fuel relay back and swapped main relay with one from eta car. No change. Actually swapped AFMs and aside from a low idle with the other one, no change. Inspected distributor cap (new last summer with the used engine), and aside from slight wear on the cylinder terminals, appeared fine. Center post resistance measured 0.0 ohms. Rotor was also new last summer with the used engine and measured 984 ohms (spec 1000+or-200 ohms). As far as I can tell, the only part(s) I haven’t changed, measured, or measured and subsequently changed are the spark plugs and the engine harness. Just didn’t feel like digging out a set from my spares tonight once 8PM rolled around.

Oh, I should also mention that cylinder compression test at the track measured within 7 psi, and I believe the range was 160-167. Either way the numbers were good and consistent.

I’m looking for any thoughts on anything I might have missed. I’m about to start poring over the schematic for anything I might have missed. Aside from a bad harness, I can’t see what it could be because I consider spark plugs pretty unlikely, especially given our earlier troubleshooting issues. Can anyone weigh in on this?

#2

Intake manifold gasket. As RP says “ask me how i know”.

#3

Bad afm can do this too. Do a search for afm with my username and you’ll find the thread from a couple yrs ago. “bad” includes an afm that has had it’s spring tension screwed with.

#4

If you do a smoke test you may get some good info.

#5

Google diy smoke generator.

#6

Thanks guys, I’ll do the smoke test tomorrow and let you know.

Ranger, I doubt it’s the AFM because it’s the same one I had since purchase and it ran great with both the old Stickley engine and the used junkyard engine. I had an AFM problem with the ETA car, which had similar (but not identical) symptoms (would always clear up at WOT). But I’ll check your thread.

Also, for future reference, this is a great resource for checking AFM’s:
http://www.the944.com/afm.htm

Note that the “early 944” is the same as the ETA AFM, and the “late 944” style AFM is the b25 and super eta AFM.

#7

What you’ll see in the thread I mentioned is that I spent the better part of a year struggling with symptoms like yours. Early in that fight I installed a fuel pressure gauge and a F/A meter on my dash, and connected both to the data logger. Sadly, it took me many months before I believed what the F/A meter was trying to tell me.

Gauges and data like that are worth their weight in gold when engine management is kicking one’s butt.

#8

Crankshaft damper bolts could be loose. Faulty wiring harness to the injectors or a bad connection at the injector harness. Those are two easy things to check that have given me high rpm misfire in the past.

RP

#9

When the engine would loose power did the tach take a big drop relative to road speed? If it did I’d say that the CPS and/or the engine harness is bad. If the tach didn’t react I’d be looking at C191 and considering the possibility that damage within the engine harness is interfering with the signals to the injectors. Oh yeah, a dodgy WOT switch is the TPS can also do this.

#10

Replace the coil…its breaking down at higher temps.

#11

Chuck, I did replace the coil with a used one I had on hand which was known good (pulled while troubleshooting the ETA car). I have another new coil I could try but doubt it’ll do the trick. The used coil never got hot because I had the hood up the entire time. Depends on how desperate I get.

Jim, I saw posts from you earlier on whether the tach dropped. I thought that the tach dropped while on track and having issues, but subsequent reproduction of the problem has yielded no tach signal dropouts. This would seem to indicate not a CPS or CPS wiring problem, right? I pulled the injector connector board and probed each conductor from the injector harness connector to the injector and sensor connectors, and they seemed ok, but that doesn’t rule out an intermittent connection issue within the assembly. The injector harness connector terminals looked fine. Is that C191? I didn’t get a chance to check my schematic yet.

Patton, I’m going to check on the crank damper bolts now.

Ranger, I’m working on fabricating a smoke machine to see what we find. No matter what it looks like a great tool that I was previously unaware of. Looks like I need to make one, then! :wink: Unfortunately, progress was slowed due to loss of my daily driver in a flood last Wednesday and dealing with corresponding issues. Hopefully I’ll have it built and in use by this weekend. Nichrome wire is on order today.

Thanks again guys and I’ll let you know what I find!

#12

If the tachometer doesn’t react you can rule out the CPS or it’s data as a suspect. C191 is the round connector under the intake between the engine harness and the fuel injector harness.

A diesel glow plug is an excellent heat source for a smoke machine and is pretty close to what the commercial machines use.

#13

So a quick round of examination yielded tight damper bolts and tight ground wire connections (figured why not, right?). Per BEdmonds, I listened for leaks. It seems to me that there was a slight hissing at idle, near the #2 cylinder intake port. Though I don’t have the smoke machine assembled yet, I tried spraying brake cleaner here and there, and thought that it might have sped the engine up after about 3 seconds when I hit the vicinity of the #2 intake. It’s far from conclusive, but I feel like it’s a reasonable indication. If so, the smoke machine should give us conclusive proof.

For those of you who’ve built your own smoke generators, what did you use for an enclosure, and what did you use for your smoke source? I see baby oil/mineral oil is common, but wasn’t sure if some had a preference for something else.

Thanks again!
Matt

#14

I have built one and used an old tank I had from some sort of sprayer or low pressure vessel (about 4" in diameter and 6" tall). The heater was a glow plug. I brazed fittings in for the glow plug and smoke output and used the existing fitting on the top of the tank for air injection. I had to hunt around to find a small regulator that could reliably work in the 2-4psi range needed. Smoke oil works best but baby oil will work.

You will also need a way to plug the exhaust and make up something to replace the AFM that has a port for smoke injection.

#15

Case of beer says it’s what i said: intake manifold gaskets. Any takers?

#16

Great learning thread for all of us.
When the problem is resolved, I’ll go old school, hit the print button, and put this in my “engine” file folder.

RP

#17

Mine is very similar to Jim’s. I bought an empty 1 qt paint can from Home Depot/Lowe’s, and drilled three holes in the top lid for the glow plug, air in, and air out. I used mineral oil.

#18

Should the smoke test not show problems, replace the fuel pump. I spent 12 dyno hours trying to tune out a high speed miss only to find the fuel pump was not holding pressure/volume properly.

#19

Ok, so I built my smoke machine and sealed my intake and exhaust. I found no observable pressure at the tailpipe, despite using a fairly substantial pressure (based on the sound and the expansion of my intake seal – a rubber glove) to supply the smoke. I applied smoke to the intake via the inlet to the air box and via the fuel pressure regulator manifold vacuum signal. Strangely, when applying smoke to the inlet to the air box, I could not observe smoke from the manifold vacuum port. Perhaps this is because the smoke appears to be heavier than air. However, in both cases, I observed no leaks from the intake manifold side (excepting the air box moisture drain hole, which I plugged before continuing my inspection). I discovered that the “bitch tube” (per Ranger’s description) leaked pretty severely at the block side (left side of engine, above the left side engine mount bracket, the vent tube between the intake manifold and the block). Other than that, I could not locate any leaks of any kind. When I pulled the dipstick, I also observed smoke escaping, so there was plenty of smoke in the crankcase. Not sure how the smoke got into the crankcase in such quantity, as I wouldn’t expect it to go past the rings so easily. Thoughts?

#20

The crankcase is connected to the intake manifold via the breather tube between the intake and valve cover. To do this test properly you must have a low range pressure regulator and a pressure gauge. A regulator & gauge rated for 0-15 or 0-20 will work and are readily available.

Commercial smoke test machines include a sensitive flow meter. Which is handy as a clue that there is a leak even if you haven’t yet seen smoke.