I built a 302 for my 87 IROC. 4.030x3.00, 6.250 rods, hypereutetic 11.1-1 pistons, 264 comp cams and roller rockers TFS195 heads and its a TPI motor. The problem is that I have a vibration from about 2500rpm to 3300rpm. It feels like a engine imbalance not a missfire. The wierd thing is that it is not consistant. Sometimes it shakes pretty good and sometimes not at all. A mechanical imbalance would have to be consistant. The rotating assembly was balanced and the whole engine is blue printed. I ve been a mechanic for 22 years and I'm pretty sure this is not a missfire. the jacobs ignition is fresh and except for the plugs the entire system was run on the old engine perfectly. The injectors are 22# and have been balance tested on the car with a pressure gage and there is no measurable difference. Any ideas, I'm open to sugestions

 

did you have it balanced with the balancer and flywheel that you are currently using?

 

Yes I had it balanced with everything, even the rist pin clips. By the way, if I had it to do again, I would not put the T56 in. It is alot of work for what you get. Serious cool factor but there is alot of good 5 speeds out for half the price and the same performance.

 

1PTR315,

I have 305 with the same vibration at the same range of RPM as you do. Maybe when you find the source you could share it with me. I have chased the gremlin for so long. I often thought it might be leaning out in that rpm range, my other thought but havent had a chance to do anything with it yet was to look at the sloppy play in my distributor. I have a little side to side slop in the distributor probably a little more than I should. So, I wonder if I have a miss at that rpm range. I know the module is good I had it tested several times and passes all the tests. I know its not drive train. I disconnected my tc and it still vibrated at that rpm range. Please let me know what you find

Ed

 

since you're a mechanic i'd scan it, put it on a scope, engine analizer, compression, leak down or whaever other test equipment you have access to. my first thought is a miss. if you used the correct balancer and flywheel and had them along with the rotating assembly balanced a miss is the logical place to start, other than someone messed up the balance work. you're using a very long rod too, that suprised me. don't have any real thoughts on that without looking into that aspect of the combo a bit more.

 

My thoughts exactly. That was the first thing that caught my eye... If you have a scan tool, any lower BLM or INT values?? Higher than normal knock counnts?

 

knock sensor and knock retard systems are working perfectly. This thing runs all day on 92 and under load (or at any other time) I have not had it ping yet. It does recognize ping on snap throttle under no laod which is one of the times the computer tests the detonation sensor. I am not very happy with my part throttle fuel curve at those rpm's and it may be a missfire after all. I'm having a hard time finding anyone who will tackel programming on this combination. The long rod is what lets me run this compression and it really widens the torque curve. This engine accelerates at 1300 rpm in 6th gear and is effortless at 6000 rpm's. I'm also getting (for what it matters) great gas mileage. I think I'm going to find the solution in programming for part throttle. The main problem is that this engine can make 350+ horse power but at light throttle it uses less fuel then the stock 305 I took out. If anyone knows where I might get programming done let me know. I don't want to spend a fortune on a bunch of chips that don't work.

 

Found it. The head gasket is leaking slowly into #8 cylinder. That is what gives it the bumping feeling of an imbalance. It is a slow leak. I put head studs in this engine and I must have one in that coner not down right. It also has a very small external leak. Thanks for the help. I'll let you know how this turns out. It will be awhile until I can get the head off now.

 

1PTR315,

May I ask how you found the leak. Was it a leakdown test? More info I am having the same problem. Thanks for any help you can give

 

1PTR315:

I'm interested in know how did you the "injectors balance test on the car with a pressure gauge".

I need check the same.


Thanks in advance,

Denis V.

 

The engine has about 400 miles on it. The first time I had to put a half gallon of coolant in did not surprise me, air pockets and all. Yesturday it was low again. Number 7 plug has a discoloration on it that none of the other plugs have.
I have a tool from Snap On that runs the injectors for a precise amount of time and openings. The presure drop on each injector should be the same. It is not very accurate but it can tell within about 10%. Injectors tend to be good or bad so most of the time it will give you the answer if it is an injector problem.

 

1PTR,

Were you ever able to determine if the leak was the intake gasket or the head gasket?

 

Yes I have a slight external leak at the back of #7 cylinder. Not even enough to drip off the engine but a definite leak. So even if the intake is the source of the coolant in the cylinder, the head is coming off anyway.

 

Why would you take the head off? Wouldnt you do a leak down test first to see if the head is leaking

 

I know the head gasket leaks, I'll check everything when it's off. I'll have the head checked for leaks at the machine shop. There is no problem with the rings so either way I will have this solved.