: Northstar Fiero + CRHFAB 272 cams with Misfires and stalls

03-17-10, 08:28 AM
I have one of my customers LS1 PCM'd Northstar's in a Fiero that would stall and misfire at operating temp.
I think I got all of the stalling issues out of the car.

Big cam tuning requires more fuel at idle. I have the car set for E10% stoich (14.1 to 1) but that may not be enough. Some tuners set the minimum injector pulse to richen the afr up at idle. I have seen many tunes that have the AFR in the mid 13's at idle.

The heads were rebuilt

His compression test was 170 to 175 across the board and the Running was about 70 psi
My compression with my VIN Y cams is the same :helpless:

I guess his cam timing could be off

New plugs, wires, and the injectors were flow tested and cleaned

SUB any thoughts

03-17-10, 09:28 AM
AJ - cranking compression looks good (hope this thing runs 93 octane or better). The running compression is a product is a product of valve overlap - what's idle speed set for ??........and did you do a running check at, say, 1,000 rpm ?

The cams turn at half speed and the sprockets each have 40 teeth (40 X 9 degrees = 360 degrees), so if you advance all 4 cams by one cam sprocket tooth (4.5 degrees at half speed) you'll move the power band down about 600 or so rpm: the cams will "come in" faster. If you retard the cams one tooth from stock the power band will move up in the rpm range, and you'll need to twist the engine to about 7,000 to get full cam power at the top of the curve. You could also do some experimenting with overlap by moving only the intake or exhaust cams............

If you have another engine, look at the cam advance/retard thing, but make sure to mock it up and roll it over with a breaker bar using a thin layer of modeling clay on a couple of piston tops to check for valve interference with advance or retard - you can do it with only one head bolted down on an already crushed gasket.

Don't forget that the Y cams are a slightly higher torque grind than 9, so there might be a tad more overlap effect at low rpm - the Y cams come in later.............

03-17-10, 09:56 AM
Manual Trans with 950 idle speed

I had 2 issues with the idle
#1 the 4.0L flow less air across the Air Flow Meter than a 4.6L. I had to rescale the air flow vs engine speed. The engine would stall after 176° (adaptive idle enabling temp) and after you opened the throttle. That is fixxed
#2 Decel fuel cut and the Clutch switch. The engine would stall coming to a stop. Turn Decel Fuel Cut off and now it is ok.

Rough idle and misfires is the last to figure out. I think is has a tight guide

04-15-10, 09:18 PM
This has been going on for about a year now and i need to get this guy handled.

Compression test
Hi AJ,

I did a compression test today.

Compression test (warm) cranking for 5 seconds and then an additional 5 seconds added to the first reading:

1—145lbs, 170lbs
2—150lbs, 150lbs
3—150lbs, 165lbs
4—150lbs, 150lbs
5—150lbs, 160lbs
6—150lbs, 150lbs
7—150lbs, 150lbs
8—170lbs, 170lbs

Compression test while Running:


Plug tips were all good, clean and white.

This looks good, but it has larger cams and I think that they should have been lower than OE spec.

04-15-10, 09:21 PM
He pulled the front cover and checked the Timing Chains.
I was thinking that could have caused the compression to be higher than what I had expected and he found nothing

Hi AJ,

As far as I can tell, the cams and intermediate shaft and crank are all in time and in the correct position as I expected. I used a square on the heads with a centerline from the cam bolt to the center of the mark on the cam gear.

I've included photos so you can see for your own observation. Pics included are of left cams, right cams and the crank and intermediate shaft position.
I have a slight bit of slack in the chain and can move it to the left a tad and to the right a tad. So, I included pictures left of center, right of center and centered.

I also discovered that there is a small port off the flapper valve or anti backfire flap at the end of the intake opposite the throttle body. Is this supposed to be plugged off? It was not plugged when running or hooked to anything in the Fiero. Would that cause a vacuum leak if not plugged?

04-15-10, 09:22 PM
I suggested a leak down test

I will do a leakdown test in each cylinder at TDC for each cylinder.

Remember though, that the misfire is random and only happens when the engine is at operating temperature and has never set during warm up. The last tune seemed to take care of it though it showed up during your present PCM tune. The heads were checked, tested and found to be in “cherry” shape according to my head shop. They said they were beautiful. That supports the low mileage 60K engine condition. So, they only did a regrind of the valves and installed new seals. That means the valves should not be tight from my perspective since no guides or bent valves were present.

The valve springs and retainers are not stock, they are the stronger set from Alan to support the 272 cams and to allow the engine to rev higher.

Another variable is with your engine used as a test. Yours is probably not a low mileage engine or the same 60K engine. That could affect compression readings when you compare the two engines.

With the engine timing correct as it is now, the only issue I can imagine is a bad hole in #1 or #8 as that is were the largest difference in readings were and because I have not done a leak down test to further our research.

One other note. I have noticed that when I push the break and the clutch peddles in at the same time, it is more prone to stall than WOT to idle when the peddles are not pushed in. Cold voltage be an issue when this is done, that is, the voltage change to the PCM when the brake peddle and clutch are pushed in?

My present thoughts/prediction is that Alans cams and the way they are ground (I assume they are ground for his purposes, his Commander PCM use and his desired tuning) are the problem. The cams may not compatible with the LS1 PCM/tune we are using because we are using a GM program. I can’t see any other reason at the moment. I’ve followed the Service manual to a “T”, your specific directions and pulled out my hair trying to figure this out on my own. So, if we can’t get this done, I’ll have to drop the cams and find another set of 300HP cams and have them reground to your specs not Alans...Or...just return to 300 HP cams after I find some and try to be done with it.

I’m starting to get fed-up with the Northstar, though I love the engines potential. It has been too problematic from the start and too long to get it on the road. Especially when compared to getting 312 HP out of my last 3800 II supercharged engine swap. It was a no brainer compared to this. So, we have to get this figured out and done. Sorry for my expressed feelings of discontent. If I had a 2 post lift, and all the special tools, it would go a lot faster but it is what it is, I guess, eh.


04-15-10, 09:24 PM
Hi AJ,

Ok. Here’s my leak down test readings. I did this just now, while engine cold, and with the engine not put together. So, that means it was not warm when I performed the test because I had no way of starting it and running it as the engine is apart.

Here’s the results using a new cheapo Harbor Freight compression leak down tester:

1: 7%, leaks into the oil pan area
2: 7%, leaks into the oil pan area
3: 7%, leaks into the oil pan area
4: 6%, leaks into the oil pan area
5: 5%, leaks past the valves into the intake port
6: 6%, leaks into the oil pan area
7: 8.5%, leaks into the oil pan area
8: 8.5%, leaks into the oil pan area

Do you have a picture of what the cam lobe position is on TDC? Are the lobes strait up or pointing out at an angle symmetrically.

04-15-10, 09:26 PM
I suggested
Pull the intake off. Then pour a little oil down on the valves. This
will let you know if it is 1 or 2 of the valves that are leaking when
you are rechecking the leak down

Hi A.J.
I did as you suggested and found both the valves to be leaking after oil was pored down on them. After seeing that, I took the head off the block and to my head shop. They went through the head and said that it was full of crap from bad gasoline.

I find that hard to believe as I had only put premium gas in from the local service station.

So, I'll strap the head back on with a new Cometic gasket (should be here in
7 days) and put it all back together.

I will use some redline fuel injection cleaner and run a tank full to clear up the other head's valves and such.

I did not find a lot of carbon on the piston tops. Nothing abnormal.

I know the intakes fill up with oil and eventually dump a little past the valves. That may be part of the cause?

Anyway, they said that was the misfiring problem most likely.

Back to the drawing board. I will put your throttle body on and run the cleaner through the valves.

I can't see that being the reason of the stalling issue however.

04-15-10, 09:29 PM
Now I will need to go back in and rework the tune.
I think I was on to the valve issue a year ago. :mad:

04-16-10, 10:31 AM
Did he ever check the manifold flapper valve tit ? That's the PCV dirty air connection, isn't it ?? Big vacuum leak if it isn't hooked up......

The compressions don't look too bad - a tad more variation than I'd like - but if there are valve seating problems, that's the answer.

04-16-10, 04:38 PM
From 6-9-09

Hi AJ,

Here's what Alan said about the 272 cam specs I have in the motor and the vacuum measurements I sent him (14" at idle and higher at higher rpms).

It looks like I have been working on this since January 2009. The flapper valve was my first thoughts. It was OK, but he did find when he pulled it down this time that the small vacuum port (used for the HVAC) was not capped off

04-18-10, 10:51 AM
AJ- this is the same issue as my 98 STS has.

04-18-10, 11:20 AM
I have to ask here maybe im not understand what this post is
but are you saying there is a little pontiac fiero like the old litlele cars from the 80s that have a northstar engine in it ?
or am i way off im just confused here

04-18-10, 11:53 AM
The "Fiero" referred to is a hand-built kit car. Several companies make kit cars which look like anything from a Ferrari to a Ford GT to an MG. They can be built to be a beautiful screamer or a piece of junk - basically it takes time, money, and automotive skill; just like building a kit airplane or boat.

This is NOT a real Ferrari 355 - it's a kit car with a WILD Chevy 327 in it.

04-18-10, 12:15 PM
oh cool cool i knew about kit cars i know how you can take a 1500 body and put a hummer kit on it and how you can turn a lebaron convertible into a mercedes body kit and the fiero you can use that for a ferrari kit

04-18-10, 01:48 PM