88Zfor_me

Hello all.
I have an 88 Iroc-z. TPI 350, Brodix heads, mild comp cam, TPIS bigmouth lower with stock runners, 52mm throttle body, hedman shorty headers, 3" single exhaust. I bought the car a couple of months ago and have replaced the ecm and done the exhuast work. I am not sure if the PROM is a factory program or custom. It does have egr delete because there is no check engine light. When I installed the headers, I took the AIR system out. As of now the air pump is just pumping into the engine bay, although it hardly seems to be pumping any air at all. This has not been deleted in the program. Any way, the car runs well and has plenty of power, but it has a strange hesitation at 2700rpm. It doesn't matter what gear I am in or how far down the throttle is pressed. As soon as it reaches 2700 rpm it hesitates as if there is too much air and no fuel. But once it passes that narrow range from 2700 to about 2900rpm it pulls fine up to 4700 where it just stops making power, I am assuming because it is TPI. Could this be a timing issue? Fuel issue?

Sorry for the lengthy post, but thanks in advance for any suggestions!

ASE doc

The power loss above 4,700 is more likely poor overall intake flow(stock runners and unported plenum?) and small cam. You don't say which Brodix heads you have either. My TPI build pulls hard to 6,200 and I shift at 6,300. Your hesitation, or misfire, at 2,700 could be a MAF sensor issue since it's one sensor that pretty much follows engine speed, regardless of throttle position. It could also be an issue in the ign module. It's odd that the engine recovers at 2,900 and that the issue is the same regardless of throttle position.

I wouldn't think that it's timing related. Advanced timing would cause pinging that would just get worse with increased throttle and rpm. Retarded timing would cause sluggish acceleration. Neither should cause trouble at one narrow speed range. A fueling issue would cause more power loss with higher throttle openings.

In a situation like this, I would start by using datastream to look for an issue with MAF sensor voltage when the problem occurs. At least since it's a pattern failure rather than an intermittent, it should be easy to track down. The AIR system being disabled shouldn't affect drivability but if you're not going to use it, you should remove it completely just to clean up the engine bay. If you're not seeing much air for the lines to the old manifold reaction lines, it's likely because it's all going to the bypass or to the line that went to the converter.

88Zfor_me

Yes. The plenum is unported, although I do have another plenum that is ported laying in my garage that came with the car. I assume the guy selling it had plans to install it but never did. That is also where the 52mm throttle body came from. The runners appear stock, although I haven't unbolted them to see if they have been ported in any way. I wish I knew which Brodix heads they were. I looked everywhere for serial numbers and such but under the valve covers all I see is 'BRODIX'. I was also reading somewhere (can't remember where it was) that there is no way to identify which Brodix heads they are without actually taking the heads off the engine. Something to do with internal measurments if I remember correctly.
Yes it is that very narrow rpm range. If I floor the accelerator I barely notice it because it passes through that range rather quickly. It is frustrating because the car seems to have an abundance of power at certain times (like within 10 minutes of being started), but it seems to diminish the longer I drive it. I just wonder if the computer is retarding timing or giving it less fuel or something. I don't hear any 'pinging'.

So is datastream the same as data logging? I have only just started learning these terms and I do not have any of the software or cables to do this as of yet. Although I plan on purchasing them from moates.net.

The only thing left of the AIR system is the actual pump. All lines and hoses have been removed. No catalytic converter either. I am planning on buying the delete pulley to get the pump out of there permanently.

I appreciate the response and any other advice you guys have!
Thanks!

ASE doc

Oh yeah. I forget your 88 has the serpentine belt so you cant just yank the pump. The fact that the engine runs good when cold verifies that it is a fuel problem. Timing doean't change with temperature, fueling doea. On cold start, the engine is run on warm up enrichment as the engine makes its way toward operating temp. You might also want to look at coolant temp as reported by the sensor(front of the manifold, below the thermostat,black wire/yellow wire) to the ECM and compare it to actual temp as read with an infared thermometer.

Datastream is a function of a factory compatible scan tool where it lets us see sensor data as the ECM sees it. Datalogging is simply recording the datastream for review later. Tunerpro, which it sounds like you're already looking into, works fine for this. The ECT sensor can also be tested using an ohmmeter. Simply measure it when fully cold and again when hot( careful not to burn yourself) it should measure about 3,400 ohms at 70 degrees(fully cold) and about 180 ohms at 220 degrees (full operating temp).

This may be an issue with the ECM's basic fuel program and the fact that your engine is modified from stock. Genrally, MAF systems are reasonably forgiving of minor modifications. I would assume that since the stock TPI base(my assumption) was used, the heads are probably no more than 180cc which shouldn't be a huge problem. The cam is mild so that shouldn't cause too much trouble either. Still, every engine management system is programmed based on the applicable engine's volumetric efficiency. Any change to the engine that affects volumetric efficiency can cause this type of rpm related performance issue. You may find a clue to your trouble by running it in the trouble area(2,700-2,900rpm) and watching O2 voltage and long and short term fuel trims. If the fuel trims go high, that tells you the engine needs more fuel than it's getting.

Something to remember when you do go to use datastream: The ECM's data refresh rate to the scan tool is slow and will cause the data as you view it to change erratically. To get a more "real time" view of data from a particular sensor or a small group of sensors, use the scan tool's custom data list feature to select just one or a few PIDs (parameter IDs) at a time.

W.E.G.

If there is a hissing sound behind your dash, you have a significant vacuum leak in your HVAC control line that needs to be fixed.

My V6 had a nasty light/mid throttle hiccup that completely went away when the vacuum leak was corrected.

ASE doc

A vacuum leak is going to cause more trouble on throttle tip in and at idle when manifold vacuum is high. A vacuum leak has almost no effect at all at WOT. It will have more effect on a smaller motor. This symptom doesn't sound anything like a vacuum leak to me.

88Zfor_me

I was kind of thinking the same thing. Maybe it is getting more fuel for warm up in the program and once it is warmed up, it leans it out. But leans it out too much to make max power at certain rpms. So is there a chance the knock sensor could send some crazy reading to the ecm saying there is a 'ping' when there is none and then the ECM retard the timing?

I do not have a scantool but I do have an ohm meter. What setting do I put it at to test ohms on the Coolant temp sensor.

The engine does have a TPIS bigmouth base so I believe it should flow a tad better and pull to atleast 5k?

Guess I need to purchase a scan tool eh? haha.

I have also checked around for vacuum leaks a while back and found one, but I replace the line and stopped it. Didn't make much of a difference in how the car ran though.

I really appreciate the replies. Kind of got my brain working again! Thanks!

88Zfor_me

I also forgot to mention that I think there is something up with the way the fans are wired. As soon as the ignition is on, the passenger side fan turns on. It runs all of the time. The driver side fan turns on once the car gets up to operating temp like it should. So I know the car isn't in limp mode. It was though when I bought it, but it turned out to be a bad ecm. After I replaced the ecm the car ran like a champ. Now I want more power!
Thanks!

oddscrounger

You're not throwing codes....

But I agree that it could be the MAF... maybe even try removing MAF and spraying it with some MAF cleaner. I was throwing a code 33 once and after I cleaned it, the problem went away.

Also, could definitely be a faulty ICM.

You say you have headers... are your ignition wires burned? Get high-heat resistant wires and zip-tie them away from the headers. New cap and rotor? New plugs?

All of those things should be checked, could be something simple. My 305 TPI would break up above 4,000 RPM and it was burned wires; I went ahead and gave it a full set of plugs, wires, new cap and rotor and wow runs great now...

That's my $0.02

ASE doc

It's so wierd that the symptom comes on in such a narrow rpm range. And doesn't seem affected by throttle position. I would have thought for sure that it was secondary ignition related if it was at all affected by engine load and/or throttle position.

The fan issue is weird. The ECM controlled coolant fan is also the AC condenser fan and it will run if the HVAC control is in any position where the AC compressor is enabled(AC, max AC, defrost) if the AC pressure switch is closed. The P.O. may have also wired one fan to run on because a relay or thermal fan switch failed at some point. The fact that the other fan runs when the engine is at normal operating temp tells me that the fan running on is probably the high temp or heavy duty fan. The high temp fan is operated by a thermal fan switch located in the passenger side head above and between plugs 6&8. If the switch or the wire to it is shorted, the fan will run on whenever the key is on.

GM electric fans are very durable and will last the life of two cars, but they were not designed to run full time and they won't last long that way. I would recommend correcting the fan problem before the fan fails. And please don't do the toggle switch fan control thing. I'll bet that countless engines have died an early death due to owners who got distracted and forgot to turn the fan on.