now it starts, but it's loping badly after warmup

[saridout]

this is a newly rebuilt engine with a rebuilt head bought off of ebay from datsun parts LLC (yeah, i know, big mistake).

first the symptoms: when i start her up, she roars to life and seems to run smoothly for a minute or two. then suddenly the engine will start loping heavily and the whole thing will jerk rhythmically to the right. if revved, it smooths out a bit, but often backfires. my vacuum gauge needle jerks into "Late Valve Timing" every time the engine lopes. on the first few startups, the loping will worsen until the engine dies. once the car has been started several times, it will lope badly but stay running. here's a video (you'll need audio obviously):

http://www.flickr.com/photos/26373558@N08/4048852266/

i've had bad luck getting a compression tester that works, but i'm pretty sure my compression is fine. i'll be testing it again this weekend. i have good spark, and all my injectors seem to be functioning correctly. there's no change when removing any of them. the car was doing something similar to this before the rebuild, but not this bad, and no backfires. i definitely did not have this kind of vac gauge reading before.

now, everything that i messed with: other than the rebore and crank work, i pretty much did everything on the engine myself.

- i pulled almost everything out of the engine bay, except the brake servo and fuel lines.

- took the intake manifold completely apart and put it back together again.

- replaced most of the hoses, including all the bits on the intake.

- replaced the starter & solenoid.

- replaced all the fuel injector clips (i may have gotten the wires backwards on one)

- replaced the 4 fusible links.

- didn't really mess with the ignition coil and resistor except to replace one of the spade connectors.

- installed head, original sprocket and original timing chain, am 99.999% positive i got the bright links right.

- replaced all major gaskets

- installed timing cover, oil pump and water pump.

-reattached manifolds, and dropped the engine back into the car.

- timed the engine.

- replaced the water temperature switch with a new one from a 260z.

- replaced a few misc spade and bullet connectors.

things i'm worried about: my engine rebuild book says that if the mark on the sprocket and faceplate are out of alignment, then the sprocket should be advanced to the #2 hole. mine wasn't nearly as off as the illustration (see attachment for mine), but i advanced it to #2 to see if it helped. it was the same, so i put it back on #1.

we installed the oil pump the way the book instructed... line up the marks on the oil pump and shaft, et cetera. the end of the shaft lined up just as indicated in the book. then, for something unrelated, we had to turn the engine a few times. i put it back at TDC, and tried to put the distributor on, and the oil pump shaft was out of alignment. i retimed the oil pump, and then it seemed ok.

i could be imagining that the above items are cause for concern, but i thought i'd mention them in case they stood out.

please help!

Edited October 29, 2009 by saridout
added video

[saridout]

here's an update:

i managed to limp over to a mechanic, and after listening to the car run, he said that i was misfiring on the first cylinder. he sprayed brake cleaner or something on the base of each injector, and when he sprayed the first and second ones, the loping suddenly smoothed out. he said i probably had a vacuum leak around one of the injectors.

i went ahead and replaced those two injectors and their seals. i just started the car up, and it ran smoothly for about 3 minutes, and then started loping again. it seems even worse than it was before.

does this ring any bells?

[FastWoman]

I was hoping more experienced hands would have a few ideas for you. I didn't really have a clue until you updated the thread with the mechanic's findings.

As your engine warms up, there's differential expansion of the different types of materials. Aluminum expands more than steel. Parts expand and possibly twist or flex. This will especially be true if you have a blocked water passage somewhere, causing your engine block to cool unevenly. (I've had that happen.) Anyway, long story short, you could be developing a vacuum leak that occurs only when the engine is warm.

What the mechanic was doing was spraying a fuel in small quantities to "sniff" for vacuum leaks. Wherever there's a vacuum leak, extra air is entering, so the fuel/air ratio is too lean. If a fuel is introduced through the leak, then the mixture is enriched, and a difference can be heard in the way the engine is running. For instance, if there's a vacuum leak early on that leans the mixture for all cylinders, introduction of a fuel through the leak will cause the RPMs to increase. In your case, there seems to be a vacuum leak feeding specific cylinders, so introduction of a fuel causes the engine to even out.

I'd speculate your problem isn't an injector leak, but rather a leak between the intake manifold and cylinder head -- or possibly a crack or defect in the intake manifold itself. You might want to keep "sniffing" for leaks the way the mechanic was doing; however, you might try using an unlit propane torch instead. Whenever you hit a leaky spot with the propane, the engine should run differently.

Good luck with it!

Edited November 16, 2009 by FastWoman

[70 Cam Guy]

You replaced the injectors along with the o-rings? Why do the injectors too?

You should try and repeat his test with the carb clean. Be as precise as you can with the spray to isolate the vacuum leak. If you can't locate another vacuum leak, double check your connectors to be sure they are all secure on the injectors and the coolant temp switch. I'm not intimately familiar with the 280 fuel injection, are there any other sensors that are temperature feedback for the ECU? Is there a cylinder head temp sensor or anything?

I have never tried a propane torch to look for vacuum leaks but it could work. It might be tough to tell exactly where the leak is

I can see logic why it's running smooth at first start. The cold start runs the car rich until the coolant temp warms up, then it leans out. Then you get the idle problems you're experiencing because the air leak creates the lean misfires.