Stroker Intake Backfire

1989 RENIX 4.7L stroker engine is about 2 years old no major problems until now.

The problem started with what sounded like a cracked header. Pulled the header there were some cracks welded it put it back on no real improvement. Drove it for a couple of days then later when I pulled out in traffic from a stop gave it some gas loud popping cracking noises and zero power. It limped home and could only go about 25mph on level ground and still making the same noises. I open the hood and it’s defiantly backfiring in the intake. So right now it still starts and idles but it sounds terrible and it probably doesn’t have enough power to pull out of my driveway.

Here is what I have tested so far:
Compression is good it’s right around 225 in all cylinders.
It’s running rich air fuel ratio is about 10. something
Pulled the plugs they were all black except for two plugs.
I have an adjustable fuel and map. I don’t remember where I set these originally they may be off. Fuel pressure is right around 30lbs Map sensor is at 4.25v

Then I tested everything else I could think of. TPS is good, coolant and air temperature sensors are within range, O2 heating element is good. The directions I found to test the camshaft sensor was to check the voltage for the blue and gray wires it’s supposed to be 5v but I was getting 7v which would be the supply voltage. I don’t know if I did this wrong.

I need some fresh ideas because I’m running out.

Since you had a backfire, check that the hose on the MAP sensor did not get blown off.

Grab a vacuum gauge and do the idle and running tests.

Throw a timing light on it and see what the timing is doing at an idle and as you bump up the rpms.

Check that the wire to the CPS didn't get fried against the exhaust header.

Sounds like a flat cam, pull the valve cover and crank it over with the coil wire off and watch to see how the rockers are moving.

I really hope it's not the cam I'll check that too.

It's unusual for a cam to go flat after being in use that long but I have seen it happen

Your MAP sensor could be bad. It can be tested. What if something in the "adjustable map" wiring or the gadget itself failed?

Mesh said:

Hose on the MAP sensor is good I even put a new one on because I snapped the old one when I was checking things.

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You are sure that you got the hose on right on the throttle body? You can get it hooked up to the wrong hole. Other than that, I would be worried about the MAP. Use a voltmeter and check it.

You could have jumped a tooth or two on the timing chain but that normally shows up on the compression test. Your compression numbers do seem to be a tad high.

I have a 99 intake and throttle body there is no port on it for the MAP. The hose is plugged directly to the intake it's worked there before and I’m 100% positive the hose is in good condition and plugged in correctly. The MAP is at 4.25v with the ignition in the on position I still need to check it while the engine is running. I've had problems with the adjustable MAP sensor before but usually the engine shuts off or stutters when it has a problem.

The engine has a compression ratio of 10.6:1 the numbers should be high.

I've also brought the engine up to TDC and made sure the distributor is indexed correctly.

I think the first thing I want to check today is the cam it's the only thing that seems to make sense right now. The problem became worse when I accelerated and would explain why two of my plugs are not black.

Remove MAP sensor adjuster and see if that changes things. Could have gone bad. Check distributor drive gear too and reindex distributor.

If the dizzy needs re indexed there is much larger problems, that should be a one time deal.

I hate to say it but I bet it's got an exhaust lobe or two that went flat, that causes intake backfire which will make the map read crazy which will make it run rich.

OR it blew the headgasket between two cylinders. This would be the best scenario.

What kind of oil have you been running?

I didn't do anything with the distributor I just made sure it was in the right position at TDC shot it the dark sort of thing.

I've been using Castrol Syntec 10w30

If your distributor is pointing at the #1 cylinder when the crank is at about 14 degrees BTDC, then your timing chain didn't slip.

I removed the valve cover turned it over and I have one rocker that barely moves. It's sloppy and I can move the push rod down a little by hand. Is this a flat lobe on the camshaft?

It's likely, how does the oil look?

Oil is clean I don't see any metal in it.

Mesh said:

I removed the valve cover turned it over and I have one rocker that barely moves. It's sloppy and I can move the push rod down a little by hand. Is this a flat lobe on the camshaft?

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Could be or you could simply have a collapsed lifter.

A collapsed lifter wouldn't take that much lift from the cam, it would still move the valve and it would make a tapping noise. The little bit he can push down on the pushrod is probably from the lifter. Either way, it needs to at least have the head pulled off. If it is a cam lobe IMO I would pull it out and tear it down to clean everything to make sure no metal is left anywhere, new cam, put it back together, and forget that synthetic oil.

Sorry Dude :/

Could be a flat lobe. Syntec shouldn't be used in a flat tappet engine. There are synthetic and semi synthetic oils that have good doses of ZDDP that are flat tappet friendly though.

Looks like I have some work to do I'll report back when it's fixed.