I recently changed my head gaskets and when I re-assembled, the engine is now harder to start than it was before and it backfires through the throttle body once I do get it started. I inspected the heads before re-installation and found no cracks and the valves seemed to be seated properly. I inspected the cylinder walls and found no cracks either. I pulled a compression test and found that it supports my theory that it is not a crack. All cylinders were uniform in reading at 120 p.s.i. I pulled a vacuum test and found that my needle drops from 10 to 2. When I increase RPM's it moves back towards 10 but, when I sustain those RPM's it drops back to 2. I think it may be a manifold leak but, I already went over the plenums and manifold. seems to be tight and that doesn't explain the backfire through the throttle body. When I got it running, I set timing to the proper setting. I even went to the extent as to pull apart the front of the engine and inspect the timing set figuring that maybe I jumped a tooth and that's why it's backfiring. It was still on mark and didn't seem to have excessive slack. Not, to mention the engine only has 15 k on it (DON'T BUY MOTORS FROM RECON). So after all this extensive research and testing I am stumped. I even asked Tommy ( Z28***** ) if maybe the fuel injector harnesses could be crossed and he said he doubts it because the harness fires on two banks ( left and right ) not independantly. As long as the left injectors are connected on the left and right on the right, he said that it should be working properly. So for some reason my spark is getting past my intake valve. And for some reason I have low vacuum. Could all this be a result of one problem or are there 2 problems? PLEASE HELP. Engine is a 2.8L MPFI

 

Strange; it does seem like a timing problem. You only got up to 120 psi on the compression test? That seems a little low to me. Did you have the ignition coil unhooked and the throttle body blades wide open for your compression test?

Did the mark on the harmonic balancer (for #1 (driver's side) top dead center) move? Bring #1 up to TDC and see if the mark lines up with zero degrees. If the isolation ring of the balancer deteriorated, it could've slipped, and thrown off the timing mark.

What happened to the car that you had to change the head gaskets?

 

did you remove the distributor when you took the heads off? When I did my head gaskets I took mine out and ended up putting it in 1 tooth off, so it was hard to start and ran like total crap. I took it out, advanced it one tooth and bolted it back down, ran perfect. Sounds like you have it 1 or 2 teeth advanced, so you might want to take it out and tuen it clockwise until it doesn't fire through the intake anymore.

 

well to answer both of your responses. I had to replace the head gaskets because one of them was warped and I figured why do just one. And If I was off by one tooth in the timing, wouldn't you be able to make that up in the cap? I didn't think you could be one tooth off because of the hexagon key pattern. Maybe 2 or 3 but, like I said shouldn't you be able to make that up in the cap? ( Why couldn't they make the friggin thing like they did V8's. you're either on or 180 out.) I have done everything I can to the distributor. I jumped the wires and jumped them back when that didn't work. I checked the timing chain. I have set the timing with a timing light...properly. I AM befuddled. If I still haven't figured it out in about a month...I'm gonna set it on fire.

 

the main questions to think about here are:

A. What could cause the spark and explosion to get past the intake valve? What could be causing a gap between the seat and valve?

B. What could cause my vacuum reading to drop from an already low reading and only increase when rpms are increased, yet drop again when higher rpms are sustained?

The vacuum thing I may have solved already but, feel free to ponder.

Just put your ideas up and I'll tell you if I know for sure that it may or may not be the problem.

Thanx for all your help and anymore you may give.

 

rocker arms too tight???

 

okay bear with me i've been drinking and I am pretty ****ed up now but hey. I set the rockers properly i think. okay i had my wife tap the key while i watched the rockers. i saw them both dip and then relax. when i saw them both in the closed position i tightened them down and then backed them off until there was very little play. that told me that they were closed all the way having play and all. i did this for all 6 cylinders. I might have the timiing off by one tooth. i have been getting a code 34 for sometime. never affected engine performance before but i don't know. maybe that has something to do with it. It's a MAF code. but, I have a new MAF. and my reference voltage checks out. maybe i'll just rebuild the 305 that just threw a rod on me and throw that in and say **** all this computer ****. wow....i'm babbling. usually happens when I'm drunk. so hey thanx 4 the N'fo. See I got some RTV gasket maker in the gasket kit I got for this engine. (felpro) They told me that was to be used for sealing the front and rear of the intake. they didn't give me the rubber seals. Maybe that's what's causing my vac problem. but what the **** is wrong with my TB. And no unfortunately I did not have my TB wide open when I pulled the compression test and I pulled the fuel fuse when I pulle the test to keep it from starting. That's how my professor taught me in school. But he may be wrong and I'm wasted so I'm gonna stop typing. Thanx for the info though guys. keeps it coming. I will be working and thinking very hard to figure out what the **** happened to this thing. I am going to buy a leak down tester soon. my boy says that if you are passing a compression test you shouldn't need to pull a leak down test. but, he's a little rusty anyway so I'm gonna do what any good tech would do. Find my vac problem and then pull a leak down test. In the meantime keep'em comin. I appreciate. \

Rocco

 

Make sure you did your valve lash right, I did mine wrong on my 305 changed it and it ran perfect.

 

.....WOW.....I was pretty screwed up.....umm.... I think what I was trying to say is that, the way I set my valves lash they should be okay. The rockers had a little play in the closed posistion. Unlike the SBC the V6 requires that you set lash before you install the plenum. You can't get the valve covers back on with the upper and lower plenum in place. So the only thing you can really do is watch your valves. They depress one after another (exhuast then intake) after that the valves should be closed and you should be able to adjust them acordingly. which I did so I don't think it's my thrush job. The code thing...I have no idea what i was trying to say there. I have had a code 34 for so long....my grandma remembers it from her childhood. I tried troubleshooting it and for some dumbass reason it stays there. I've checked everything pertaining to that code and got nothing. Never affected performance though. I am going to take a peek under my hood today and just kinda double check alot of things. See if I can find any clues. I'll keep you guys posted.

Thanx

 

P.S. next time I leave a post when I'm drunk...contact a moderator. Tell them you find it offensive or something. There is no need for me to be senselessly rambling on. Nothing really useful was said, I don't think. Plus it wastes space. I am so p-issed at myself. I have no time for my own games. I have a real problem here that I am trying to get fixed and I'm leaving posts when I'm drunk. People won't take "sober me" seriously if "drunk me" gets on and starts with the mindless babble-yak. So do me a favor peeps. Next time, if that happens, get a moderator in here and have him delete that crap or something.