It's been nearly a year but I'm prepping my racecar for my next (annual) lemons race in March. One of my teammates came in this weekend after finishing our first DIY alignment (I had to think back to my HS trig as well to calculate total toe) and finishing up a brake overhaul, I decided to tackle the surgey idle we experienced at our last race. While the car felt smooth at WOT, it would buck a little bit at partial throttle in the pits (as described above). FWIW, one of my teammates also felt that the car felt a little down on it's power at the end of day 2 but that could have been chalked up to driver fatigue and/or getting used to how the car drove (laptimes generally improve over the course of the race weekend given how many laps we'll do on the track + the field opening up).
Well, we found our problem. For other's benefit, here's what we did:
Test 1: Vacuum test
- Measured 10 HG/in at idle. When you open the throttle, it would respond quickly and went up to 20 HG/in.
- MadMikey says this was a bit low since an e303 cam should be from 14-18 HG/in in his experience so it merited further testing. At this point, we thought it was a vacuum leak.
Test 2: Looking for a vacuum leak
- Originally I tried an unlit propane torch but we didn't see a rise in RPMs even with my tank fully open and aimed at a vacuum opening. Instead, we used the brake parts cleaner sprayed on the various gaskets.
- Results: nothing found. I was hoping it would have been an intake gasket leak.
Test 3: Compression Test
- Most cyl had 110-115 PSI (within 10%) but #2 and #3 were at 50PSi and 85PSI respectively.
- I seem to recall compression testing and getting more like 145PSI so this had me a little torried.
- The two cylinders with significantly lower compression ratings was a definite red flag.
- Squirting oil into the problem cylinders didn't improve the test numbers so the issue wasn't (likely) with the rings.
- Possible root causes at this point: blown head gasket or valve issue.
Test 4: Pressure test
- This one I have MadMikey to fully thank for suggesting this test. The idea is to inject air into the cylinder when it's at TDC and then look to see where it is leaking. Possibilities include a spark plug hole in an adjacent cylinder (because of a head gasket leak), into the oil pan (BHG), into the oil pan (rings), or finally into the intake or exhaust tracts (valves).
another (head gasket
- Since we knew the problem was at the heads now, the goal was to figure out the exact point of failure. We went ahead and took the upper/lower intake off. We also took off the exhaust manifolds to directly expose the exhaust ports to the outside air.
- Our compression test tool had a hose that screwed into the spark plug hole and could then be attached to an air compressor. NOTE: We did had to remove a shrader valve (a one way valve) at the end of the hose that screwed into the spark plug hole though. This allowed the test gauge to maintain pressure a certain pressure until you pushed the button on the gauge to release it.
- We moved the problem cylinder to TDC to make sure the piston was fully raised.
- After pumping up the air compressor's tank, we raised the operating PSi from 0PSI to ~40PSI and then listened to the engine to see where the air would go.
- We found that for the two problem cylinders, we were hearing compressed air escaping into the intake tract. This meant that the intake valves weren't fully seating and exhaust gases were being reintroduced into the intake tract. THAT caused all of the initial bucking experienced at low RPMS as the whole air/fuel mix would be different.
- Just to confirm things, we removed the rocker arms so the valves should have been stuck shut and tapped them with a rubber mallet to fully seat. Air was still leaking.
CONCLUSION: valves not seating were the root cause of our performance/throttle issues.
- At this point, we realized that we needed to pull the heads and take them into a shop.
- Once the heads were off a car, we shined a bright LED into the suspect intake tracts and you could see a little light leaking out from the edge of the problem valves. FWIW, no issues were seen with the head gaskets (you couldn't see any spots where exhaust had eaten into the ring around the cylinders.)
- We also inspected the block and the cylinder walls show no signs of scoring or and point where the valves hit the piston heads (whew -- they have reliefs on them but I still have flat top pistons). For now, I'm not expecting to do any further block work (other than draining the oil).
- Off to the machine shop they go for valve work and to confirm that it's not cracked (magnafluxing) and properly flat. The machinist might need to replace the valves if they are bent, regrind the valve seats, or replace the valve seals. Either way, the shop will do this work.
- I realize now that in the first race when I first rebuilt the V8, the engine got really, really hot (insufficient cooling system at the time). This probably warped some of these valve stems or messed up the valve seat.
- When i rebuilt the engine the last time, the heads were allegedly "checked out" but I probably should paid to have the heads inspected more closely and more thoroughly gone over.
- While we were able to finish our last race on the same engine we started with, we clearly haven't been able to go between races without opening up our engine yet
At least our suspension is totally baller now (I can jump on the fender and the cars nose barely goes down. Jumping up and down on the fender makes the car barely move. It's not going to be comfy to drive it on the track but it's gonna be fun).