Engine cutting out around 4100 rpm. Need help

[brent8487]

I'm new to the board, but been lurking for years now, and really enjoy this board. I have an 84 GN with a 87 drive train I installed in 96. I restored the car two years ago, including a mild engine rebuild. I have about 3000 miles on the car since the restoration, without much trouble until recently.....

When I put a load on the car, (driving) around 4100 rpm the engine cuts out.
I get some knock, 1-6 on the scan master, when the stumble occurs, acts like a rev limiter. Never get a check engine light, the light does come on if I unhook a sensor, so the light is working.

I had the stock MAF go out on me about 8 weeks ago, replaced it with a 3" and translator, car did fine for about 100 miles then the stumbling started.

Here is what I've done with no change to the problem,

Replaced:
Crank sensor, ring looks good, gap at .025".
Fuel filter.
Walbro 340 pump, added hot wire kit. Old pump was a Red Armstrong, about 8 yrs. old.
Upgraded to 60#motrons W/ TT chip. Used, bought from a good source (also reset switches in translator box, reset computer)
Spark plugs, old ones looked good gap at .033" AC43ts
Plug wires


Swapped parts from known good parts:
Ign. module with coil pack
computer
ESC
MAF, a 3.5" then a stock one
Cam sensor cap
02 sensor

If I rev the engine fast and in park I can duplicate the stumble at 4100rpm.
If I rev it semi fast, it revs fine/normal, scan master numbers look good.

Here are my numbers from the scan master, with the car in park.

At Idle:
02-varies-0 Kr
AF-06
LB-38-41
Bat 13.8
Int-125
BL-110
MPH-00
CLT-160
Ats-116
R-825
TPS-.42
IAC-10
CC-climbs then starts over
Mal-00

Duplicating the problem in park, with a quick "stab" to the gas:
02-varies, 200-700 (when stumbling) KR-2-7 (when stumbling) will climb if I don't let off
AF-140
Lb-200
Bat-13.5
Int-128
Bl-124
Mph-00
Clt-168
Ats-136
Rpm-4250
Tps-3.64
Iac-53
CC-16- will climb
Mal-00

I've checked my air duct work, no holes.
I recently TIG welded by exhaust manifolds.
Compression check is 150 psi on every cylinder.
Unplugged computer wire, cranked engine with valve covers off, rockers are all moving the same distance, (by eyeball)
fuel pressure is 43# with line off, I've adjusted up to 50# down to 30#
Fuel pressure increases with boost 1to1.
Have good grounds, have a wire going to left fender to bell housing, to A-arm bolt. Also have gas tank grounded to frame.
All vacuum hoses replaced at restoration. 15 Hg. in. at Idle
Boost gauge and Alcohol is tapped off the line by the left side of coil pak.
Egr was filled when rebuilt. Heads from Champion.
Have HanR motor mounts. Looked all over for anything rattling causing a false knock.
Looked for shorted, open wires, can't find any.

I will accept any ideas and will try anything to remedy this problem.
I appreciate your efforts,

Thanks.
Brent

Also, If anybody has a set of center caps for the stock 84 rims, I'd be interested in buying, someone stole mine off my car when it was in my shop.

I hope it wasn't someone I know....

 

[Just a Six??]

Something to look at related or not?? Caused me some headaches a while back. The new translator harness that connects to the stock harness you need to check the pins & connections as mine were not making decent contact & sometimes it ran fine & other times really messed up. Took a Long time to find it but then at the Full Throttle Tech site it was mentioned & sure enuf was the problem. Again, maybe Nothing to do with you but good to take a peek anyways for the 2 mins it takes. Good Luck!

 

[brent8487]

I've looked over itbefore, but not up close and detailed.
I will go over it again (closely), could be the problem...

Thanks.

 

[brent8487]

Translator connection

Connection looked good, traced the MAF wires as far as I could into the engine loom, looked good. Even swapped out the MAF sensor, pipe and translator with a known good one, still the same problem.

 

[johnplogii]

I had one acting similar (miss at 4100-4200) and at a quick glance the rockers were all moving well, opening & closing the valves, but upon closer inspection the two on #4 were only opening about half way. I actually rested my finger on them while running to be able to tell they were not up to par. So in my case it was a partially flat cam on two lobes.

 

[brent8487]

I'll give the rockers another look.
I bought the cam though the GSCA. I explained my problem (thinking the cam was the issue) to them and asked them if they had any issues with cams, their reply was no one had reported any cam failures. They use Reeds cams. Could be the problem....
I'll look into it today...

 

[brent8487]

Cam seems okay

Took the valve covers off, ran engine at idle, "felt" every rocker with my finger tips, everything seemed normal. I did not put a dial indicator on though, should I?

I've witnessed flat cams before, (although on Chevy engines) and a flat lobe was obvious. Maybe the Buick's are different?

If a flat cam was the issue, wouldn't I notice the problem consistantly?
Don't know, just asking...

I lean a way from a mechanical issue, because of the intermittent problem, or a fuel issue, with the new parts and numbers from the scan master, but am still open to any suggestions....

 

[johnplogii]

It was just a suggestion because I tore my hair out on the car that did that. The cam was not totally flat, only about 1/2 the normal lift. Totally flat would show up alot easier yes, but it sounds as this is not your problem. The one I am referring to I could hold my finger on the two rockers in question but the others would all kick my finger right off. You had to look real hard from the right angle to see that they were not opening as far as the rest. Good luck with it, it sounds like you have exhausted all possibilities on this one.

 

[SPOOLFOOL2]

Not sure how much boost your running, but have you tryed gaping your plugs down to .028"? When it "cuts out", does it sound like it's popping up the intake, or out the exhaust? Does it pull strong up untill 4100 rpms? Could be a bad valve spring(s). Check the seat pressures. Springs can be changed with the engine in the car, if you know how. If a cam lobe was going flat, you'd have metal in the oil. It would probably idle funny too.
Good luck.

Mike Barnard

 

[brent8487]

Johnplogii, thanks for your sugestion, I haven't ruled out the cam completely, but from what I can tell the valve train is o.k.

spoolfool2, I've turned down the boost to 10 psi, so not to "strain" things too much. Like i mentioned above, I can duplicate the problem with the car in park, at no boost. The engine has never seen more then 16 psi boost, even when things were good.
Not really a "popping", engine keeps trying to rev, but something is cutting it off, like ingnition, or fuel.
Car pulls strong to till around 4100 rmp, at that point I maybe in about 5-7 psi boost. (hard to watch all those gauges at once)
Valve springs looked good when watching them in action. Have about 3000 miles on the engine, heads are from Champion. Anyone know if the've had spring problems? Could be?
Idle is very good. Everything is good to about 4000rpm with a "stab" of the throttle. If I rev the engine a little slower, it will redline with no prolem.

I havn't tried gapping the plugs to .028". Will do tommrow, It's dark out now.
If the problem goes away, would that indicate a possible weak ingnition?

Thanks for the advice Johnplogii and spoolfool2

 

[brent8487]

Regapped the spark plugs to .028" still have the same problem.

Any other suggestions???????

 

[johnplogii]

Have you checked the throttle body/plenum for possible oil from pcv? A lot of oil could do that too.

 

[brent8487]

Took the upper IC pipe off, looked inside throttle body, blade open, just a very light film of oil, normal I think. Left PCV off plugged vac line, let engine warm up, still same problem.
I haven't seen any oil consumption. Only time I see smoke is when I let off the throttle after the engine stumbles (car in park) the smoke smells rich.

Thanks Johnplogii

Any other ideas?

This is a tuff one. Will probably be something simple.....I hope.....

 

[SPOOLFOOL2]

Typically, Ignition problems will get worse with load (boost). This sounds like it's rpm related. Did the plugs all look the same? What kind of cam and lifters are in the engine. Comp has had lots of problems with hydralic roller lifters not pumping up lately. Still could be a weak valve spring.

 

[Sam Colalillo]

What is the seat pressure on the valve springs?

 

[brent8487]

Spoolfool2, I agree it's rpm related. Plugs looked the same, #3 may have been slightly lighter in color, so if I have a spring issue, I'll suspect #3.
Cam was bought through the GSCA, 208 Reeds cam with GM hydraulic lifters.
How do check for a weak spring?

Sam Colalillo, Don't know how to check seat pressure.
I bought the heads off the shelf from Champion, have about 3000 miles on them, and engine, haven't had them off since the engine was built.
How do I check seat pressure?

After work I'll do a search on this forum for checking seat and spring pressure.

Thanks for all the input!

 

[b_rich24]

A couple of things to definitely check for your issue: 1. follow your positive cable from your batt to your starter, ensure there is no break or exposed wire grounding out on either your header or your starter heat shield. This will cause it to stumble under load (usually giving a SES light). 2. check your tensioner. If it is worn, it will slip under load (no SES light).

I'd check these two before tearing down/removing anything.

 

[87NAT]

My car had a stutter/miss at high rpm. Turned out that the cam was worn down...didnt lose a lobe...just worn down. I pulled my hair out for months(and emptied my checking account) trying to find the problem. BUT...the problem went away for a 100 miles when you change to translator


That doesnt indicate a camshaft failure. I will be watching this thread to see the progress on your situation.

 

[Sam Colalillo]

Sam Colalillo, Don't know how to check seat pressure.
I bought the heads off the shelf from Champion, have about 3000 miles on them, and engine, haven't had them off since the engine was built.
How do I check seat pressure?

After work I'll do a search on this forum for checking seat and spring pressure.

Thanks for all the input!

Only 3,000 miles on champion heads, I quess I would rule this out. THere is a tool (Moroso part number 62390) which latchs onto the end of the rocker shaft, and has a scale on it. When you pull back on the tool it moves the needle on the scale. Keep pulling until the spring moves, record the amount of LBS force it took to make the spring move.

 

[brent8487]

I didn't have the opportunity to look at the car today (sure wanted to need to get past this problem) got busy with other things, I'm sure you all know what that's like.
b_rich24, I will try your suggestions tomorrow (hopefully). Thanks.

87NAT, the MAF sensor went out in June, installed the 3" MAF and translator, car was fine for 100 miles after that (and was before), then the stumbling started. I installed my alcohol kit and Art Carr trans a few weeks before the MAF went out.

Maybe I need to look at my timing and be sure it is advancing. And/or put a dial indicator on each rocker, to verify proper cam movement.

Thanks everyone.