Gummie

I've been having an intermittent problem with the car running into a brick wall at about 4,000 RPMs at WOT that’s recently gotten worse. It’s fine if you walk the car up past 4,000 but the instant you nail it the engine acts like it’s hitting a rpm limiter and feels like it fuel starves its self. You can take the car all the way to redline in gear at part throttle though. If you stay on the gas after it happens the engine will rev down to a certain point around 3,700 and then it will act normal again (ie rev quickly back up) to around 4,000 and then do it again.

The tack doesn’t fall so I’m assuming I have ignition. I hooked up a fuel pressure gauge and drove around and pressure wasn’t falling off so I’m assuming I also have fuel. Fuel pressure was fine @ idle and pressure would gradually work its way up a few more pounds as you get on the gas then it would level out by the time I hit about 3,000. The needle didn’t budge when I hit the 4,000 limiter thing.

The thing to figure out is this: what sensor input would cause the engine to lose either fuel (injectors shutting off) or spark at 4,000 RPM (it’s almost always exactly at 4k) but only under load?

I assumed MAF and replaced it yesterday. The car drove perfectly after that. I thought I had fixed it. I drove to Princeton in the rain later that afternoon with no issues. The next day I took a friend out for a ride and it did it again right at 4,000 RPM.

I have a functional ram air hood. Is it possible that the water killed the brand new MAF? Did I just get lucky on the way to Princeton since the problem is intermittent? Anyone have any ideas?

Red Devil

I have a serious problem buying the MAF as the culprit in the first place. :shrug:

89Vert

whats your distributor cap look like?

i've seen problems that occur at exact rpms cause the spark is tracing in the cap

88 IROC CONVERT

change fuel filter lately? sounds like its starving up top

jjb

I have seen weak ignition coils cause a similiar problem. Also, if the coil wire is bad, it can cause weak spark that only shows up under high load situations.

Gummie

The filter is about 3 months old and I usually only run premium. It is a daily driver so it sees some miles but I would like to hope this isn't the problem.

I will pull the cap later today to take a gander.

Is there any way of testing the coil without replacing it? I hate to say I'm on a budget because I'm usually willing to just replace stuff but I'm out of a job for another two or three weeks while they repair fire dammage at work

It still boggles me that it will only do it under load.

88 IROC CONVERT

THEN INGNITION SOUNDS LIKE THE TICKET!

Red Devil

Before swapping out parts, pull your ignition module and have it tested. Don't forget the correct compound for under the module, dielectric isn't meant for it. If you need a part number look in the 'accel module' thread in the aftermarket review board.

92 Formula

iTrader: (2)

This RD's post that he mentions above:

Gummie

Thanks for the heads up on the ignition module. I'm going to take a DMM to my throttle position sensor tomorrow when the sun is up to eliminate that off the list.

Where can I get the module tested?

DENN_SHAH

iTrader: (1)

first thing i would do is hang a gage & check fuel pressure when it you nail it.

if you have the external coil, pull the coil wire off at the coil & have someone crank it over, if the coil is good it should be able to fire down the side of the tower to the laminations

Gummie

I duck taped it to my windshield and drove around. Fuel pressure would inrease from idle pressure with RPM and remained constant (no spike or dropoff) when the limiter thing hit.

Gummie

small update: checked the TPS. .48 volts closed 4.3 WOT. No dead spots. I'm going to try to adjust it to spec (0.54Volts +/- 0.075Volts) closed and we will see what happens today in my driving.

Gummie

Another update: it basically has to be the MAF.

Earlier today I adjusted the TPS to no avail. Then I talked to one of my friends about it. He told me that since I was running a pretty old computer I could just unplug things sensor by sensor (MAF, TPS, coolant temp, etc) until I found the culprit. TPS checked out fine. I plugged that in and unplugged the MAF sensor and the car ran right to redline in first.

I guess the reman was a POS and I managed to kill it in a day. Damn expesnive reman to at $220.

Gummie

Just got done installing the second MAF. Still no luck. I checked my wiring extension and everything checked out ok (had to extend the wires going to the MAF for the ram air intake). Everything measured @ .6 ohms or less. There was one weird thing though: the black wire and the black/white wire were continuous. Anyone know if they’re ground wires?

I’m going to go back to step 1 and unplug the sensor again and see if I can still get it to redline in gear. Maybe I got lucky last time?

If anyone has any additional ideas I would greatly appreciate it.

Red Devil

MAF relays. Lookj around there are a few threads going right now about MAF problems, and I think those two wires may be grounds, I don't have a schem in front of me.

Gummie

Doesn't the MAF relay just burn the crud off the wire? I would think a new MAF would have a clean wire, even if it is a reman. I will look for the other threads though.

Gummie

Where are the relays located? I did a search but didn't find a ton of info, only that the two relays are two different part numbers, autozone only lists one number, and it doesn’t physically fit in both spots.

I got the complete wiring diagram for the engine bay and there's only one wire that actually goes to the ECM from the MAF other than a ground reference and it looks physically in tact in the car. I will check resistance through that wire tomorrow but it should be fine.

I'm really at a loss. I tried unplugging the sensor and driving around today and it was perfect aside from not running 100% for obvious reasons. This means it’s either the MAF circuit or the computer does something with the MAF unplugged differently than when it’s plugged in. The WOT thing still bothers me. I almost think it has to be ECM related?

If anyone has any kind of advice I’m more than willing to listen.

Red Devil

Photo courtesy of Vader:



Many are the same (power and burnoff - I've never actualy seen those two not ne interchangeable, but I've heard the rumors), some use the same for the MAF as the fuel pump as well.

The difference is that the computer goes to predetermined tables in its program when it has a major fault such as a non-responsive MAF. If you want some good reading do a search with MAF and user name Vader. He's written more that I have.

Gummie

Nice picture. I can tell you visually that my relays are different for the power and driving lamps – mine are somewhat oval looking, like the fan relays for the dual cooling fans. I know it's getting power because I swapped those two. The burnoff relay might be bad though - I've got nothing to swap it out with. I wish there was some way of testing it.

Wouldn't unplugging the TPS count as a major fault? I know that sets a code and turns the check engine light on. The engine still falls on its face at 4,000 even with the TPS unplugged.

Vader

At 4,000 RPM (or whatever RPM) with the throttle open 80% or more, the ECM may not even be looking at the MAF. Well, technically, it would be monitoring the signal, but not using it for fuel calculations. Once the MAF is maxxed out for flow, the ECM is only fueling based on tables, not the MAF input. You may have modded the engine enough and improved VE to the point of being beyond the capability of the stock programming.

If you don't find the problem in the ignition, you may want to consider that possibility.

Red Devil

The pic is of an '86 set-up. My last '89 were all oval and both the MAF relays and the f/p relay were the same. :shrug:

Gummie

edited:

That's exactly the kind of answer I was looking for. I'm willing to be that the engine hasn't been modded to get past the point of the stock programming. The problem has been gradually getting worse over time (no additional mods) so it makes sense that a part is failing. It could be ignition related but I've got no idea why it would suddenly be better with the MAF disconnected?

What ignition related things should I be looking at?

Red Devil

Hmmm... looks really familiar... I take it you never did it? You have two projects for tomorrow.

Gummie

I forgot about that. It does indeed look like I will have something to do tomorrow. I guess for now I will stop worrying about the problem vanishing with the MAF unplugged.

This brings me back to my other question:

Or how can I test it myself?

91RS5speed

I had a simular problem with mine even. I dont have a maf though but my o2 sensor had backed out and wasnt getting the right signal and was dumping to much gas in at 4+k. It seemed like it was hitting a rev limiter.

3GTAs1TA1Z28

I Know exactly what you are talking about....I recently had the same issue as you but fortunately for me I own 5 of these cars and had plenty of spare parts to test with.....I replaced everything.....

For starters a few specs
350 .40 over
High Rise Cam
69 Fuelie Angled Plug Heads 2.02 1.60 Valves
SLP Ported and Polished Siamease Runners
Ported and Polished Plenum
......
Car ran like Brass then one day went to punch it and like you said it hit a wall just like a rev limiter....

Thought MAF sensor...Swapped in a New One had still in box no help.
OK next Pulled MSD 6AL Box....Still no help
OK next Swapped in new Ignition Control Module
Still Issues so I then swapped coils nothin
Next camp ECU still no help... Then Prom Nothin.
Then IAC and TPS Nothin
Checked my Holley AFPR and my Walbro right at 50 where set
ok Checked timing dead on....
New Cap and Rotor Still problems...
OHM tested my MSD Wires they were fine...

I was running out of ideas....
Then swapped MAF Power and Burnoff Relays No help
Tried the ESC Still no help


After all that it boiled down to a bad connection from a wire splicing i had done between the Harness and the ICM.....I pulled it apart soldered it back together shrink wrapped it and have been running like a champ ever since.

Gummie

As far as I know there's nothing spliced/soldered by the ICM. I spliced on one of the two molex connectors going to the coil because I broke a pin in the orginal but the problem existed before hand.

Where exactly is the ICM?

91RS5speed: thanks for the idea. I will check that as well.

3GTAs1TA1Z28

ICM = Ignition Control Module...You know the one at the base of the Distibutor has 2 connectors in it one to the harness one directly to the coil.......I would make sure you have a good Solder Connection there at the splice you did and not just some but connectors.....That was the same area I had issues

Red Devil

Parts store can test it for you. If it tests bad, have them test the new one before you leave, they have been known to sell bad ones right out of the box.

purpleZ-28

i had a bad ignition coil the day before i turned 18. i was out downtown, lookin for at tattoo parlours, and when i got to the car, it wouldnt run, it just kept dying. so i pulled hood off, couldnt figure anything out for the life of me till this guy came out that worked in bar that was next to the parkinglot. and while my girlfriend cranked it, the bartender and i stood in awe as the coil wire arced against anything within an inch of it! so we put the hood and called a tow truck. once we got it home my dad wrapped the coil wire in electrical wire and it fired right up. so try the electrical tape, and if that doesnt work, yo can get a coil wire from autozone for $5. i know, because i did that the next day.

Gummie

Thanks for the info guys. I'm not going to have a chance to do this today because I've got too much stuff going on but hopefully I will be able to get to it over the weekend. I will keep you guys posted.

Thanks for the help/advice so far

edit: purpleZ-28 - the car starts/runs fine. Well, it is starting a little rough but that's no the main concern right now. My problem lies at 4,000 RPM at wot. I will be looking into the coil though.

3.8TransAM

HAve u tried cleaning all the grounds yet?

Most ecm grounds are on the pass side rear of the cylinder head.

Might want to make sure they arent broken or extremely corroded.

Seen similiar problems from it as well.

later
Jeremy

Gummie

Being a stereo nut I upgraded my grounds to 2 gauge. I've got 2 gauge running from the chassis to battery negative to the engine block. The stock grounds are still in place as well. The grounds defiantly helped things electrically (faster start, higher voltage) but they were done well before this problem developed so I can’t say their impact on this.