Help me diagnose a problem

[E30Nova]

So I bought a Mark D chip for 19lb injectors around 6 months ago in a group buy and ever since I installed it I've been having random WOT problems. When I floor it, sometimes it will accelerate fine and other times it will accelerate and then cut out like I slammed the brakes. Then a half second later it starts pulling again.

I've tried making threads about it a couple times on bf.c and nobody seems to know what it could be.

I've tried swapping back to the stock chip and it still did it, so that rules out the chip. I fixed every vacuum leak I found, made sure my injectors were plugged in correctly and checked my grounds.

Then, a few weeks ago, someone said it was probably my TPS. So I tried checking it (unfortunately my super amazing DMM was stolen, so I only have a $3 Harbor Freight DMM) but my DMM doesn't do continuity. I tried putting it on resistance and opening the throttle. That kind of worked. When I opened the throttle the first time, the resistance changed at WOT. So I closed it and it changed again. Then I re-opened it and it didn't change. I don't know if that really means anything, but wouldn't that indicate my TPS is shot?

Aside from that, is there anything else that would cause my problem? I tried emailing Mark about it, but once I told him I bought the chip in a group buy, he stopped emailing me back. :(

Matt

[slow e30]

this is the SAME problem i, and a few others faced a year or two back. a few of us dumped countless bills into the engine to fix the problem. i never found the solution, and i thought i was cursed.

check with a known working TPS, and a ICV.

[E30Nova]

Interesting. I've searched the main E30 forums (bf.c, r3v, and e30tech) and googled every possible phrase I could think of and I only found one other person with this problem. His solution was that his spark plug wires were worn out to the point of arcing and cross-spark. Mine are brand new, along with my plugs, so that's not my problem.

What I don't get is why it does it with the stock chip. All I did was pull the fuel rail, install the new injectors, pull the DME and install the chip. I've built a few computers and have modded some consoles, so naturally I grounded myself and even wore a ground strap, so I'm basically 100% sure I didn't fry my DME. I didn't touch any vacuum lines or my TPS, so I don't see why that would suddenly be a problem.

I'm about to just buy the Miller MAF and hope that fixes it. I'm tired of dealing with it. :x

Matt

[Quadrajet]

Did you run a pressure and delivery check on your Fuel pump/ Fuel Pressure Regulator? Seeings how you've covered a bunch of other stuff but didn't mention the fuel delivery system.

[nando]

if it's cutting out like you describe that can only mean a few things - the computer is getting a synch loss from the crank sensor (and cutting fuel), or it's cutting fuel for some other reason (bad TPS could be it if the computer sees idle suddenly even when you're at WOT).

the other option is a bad relay or something else shutting off your fuel pump.

When it cuts out, does the tach momentarily drop to 0 or does it keep up at the same RPM? does the check engine light flash at the same time for a split second?

[E30Nova]

No, I haven't done any fuel tests. It also failed smog recently, so I just kind of got pissed and parked it. I miss driving it now, though (frickin Nova gets 17 mpg :down:) so I'll probably do some fuel tests some time this week.

if it's cutting out like you describe that can only mean a few things - the computer is getting a synch loss from the crank sensor (and cutting fuel), or it's cutting fuel for some other reason (bad TPS could be it if the computer sees idle suddenly even when you're at WOT).

the other option is a bad relay or something else shutting off your fuel pump.

When it cuts out, does the tach momentarily drop to 0 or does it keep up at the same RPM? does the check engine light flash at the same time for a split second?

It's been a while since I've driven it, so I don't remember what the tach did. I DO know, though, that the check engine light always comes on when it loses power. Of course, when the power comes back, check engine light goes off. That doesn't really help since OBD1 doesn't have pending codes, so every time it happened and I scanned the car afterwards, it said everything was ok.

Matt

[nando]

do you have any coolant leaks around the throttle body?

I bet your tach is dropping to 0, but drive it a bit and let me know. Motronic 1.3 may not have many diagnostics codes, but it's so simple there really isn't a lot going on. You just have to know what it's expecting. :)

[E30Nova]

do you have any coolant leaks around the throttle body?

I bet your tach is dropping to 0, but drive it a bit and let me know. Motronic 1.3 may not have many diagnostics codes, but it's so simple there really isn't a lot going on. You just have to know what it's expecting. :)

No, but I had to JB weld those fittings on the right side of the throttle body (the ones that go to the brake booster) since they broke. They could possibly be leaking, but I hooked up a smoke machine to my engine and no smoke came out of there so I'm pretty sure they're good.

Matt

[nando]

so you had a leak there in the past or?

one thing that can happen is a leak there will drip onto the round injector harness plug under the manifold. Eventually it can get wet enough to corrode or short out. I'd have a peak under there to see if you can detect any signs that it is or ever was wet. This is what happened to me the second time I got the engine cutting/check light/tach dropping to 0 issue. It went away once I fixed the leak and everything dried out, but it didn't have a chance to corrode.

the first time it was the CPS wire rubbing on the water pump and shorting out.

speaking of, have you tested your CPS?

[E30Nova]

so you had a leak there in the past or?

one thing that can happen is a leak there will drip onto the round injector harness plug under the manifold. Eventually it can get wet enough to corrode or short out. I'd have a peak under there to see if you can detect any signs that it is or ever was wet.

also, have you tested your CPS?

I meant vacuum leaks, sorry. My memory is pretty bad, but I'm pretty sure there was a lot of grease and/or oil on the TPS. Just sayin. ;)

I remember doing something to my CPS when I first got the car...I don't remember what, though...:(

I wanted to borrow my friend's DMM to test my TPS tonight but he couldn't find it. If I don't need to run a continuity test to test my CPS, I'll go do it right now.

EDIT: I just googled how to test the CPS (for some reason I couldn't find it in my Bentley) and I think I tested the right connector. There's two by the diagnostic connector and I tested both of em. The one closest to the driver's side had 546 ohms between pin 1 and 2, and 1 ohm between pins 1 and 3, and 2 and 3. The other one had like 216 ohms between pins 1 and 2, and 1 ohm between the other pins.

Matt

[nando]

okay, what I was asking about specifically was coolant leaks. the TB heater has a cork gasket that will start to leak as it ages (or if the hoses are old, it will leak out of there).

a vac leak isn't going to cause the issue at hand. :p

don't think grease or oil really would anyway.

Actually, there is one test you can do that's pretty easy. disconnect the TPS and drive the car around for a while. When I had the issue of a wet harness, this solved the problem well enough for me to drive the car until I fixed it. if it stops cutting out then it's a wet harness. if it still does it it's probably something else. you'll be running on the cruise maps all the time though so it won't idle nicely and it will be slow..

[E30Nova]

Well I disconnected the TPS and tested it. Tested fine. Problem is, I broke the stupid fitting on the throttle body that has a hose going to the brake booster. I tried to JB weld it but you can hear it sucking. I put my finger around it and the sucking noise got quieter. I might try slabbing some more JB weld on it, but if that doesn't work, I might have to buy a new throttle body.

I also changed the SI board with a known working one (my batteries were at 2v and the one I put in had 3v) to fix my gauges. Now my idle is at 1500, and I'm pretty sure it's because of the vacuum leak since I used my original coding plug on the SI board.

Is there another way to fix those fittings? I remember reading somewhere that there was a "kit" to put new fittings in from the dealer.

Matt

[E30stan]

i had similar symptoms with my car, i was so sure that it wasnt a vacuum leak until i went to change my tps and noticed that small rubber tube under the throttle body had a tear in it right under the clamp, so sometimes it would allow extra air in causing the slam-on-brakes feeling, eventually it was fixed but then the o2 sensor went bad before i fixed it in time

[E30Nova]

Well, I finally decided to get off my ass and try to diagnose the problem. I just bought an afm and throttle body last night. I haven't put the throttle body on yet because I'm too lazy to drain coolant, but I put the new afm in. I did the fuel pump test, though. Jumped the fuel pump relay for 30 seconds and only had 870ml, so I didn't even get the bare minimum according to the Bentley. I'll probably do the test again just to be sure, I mean is 5ml really going to make a massive difference?

I tried to do a fuel pressure test but I used a vacuum/fuel pump combo gauge that only went to like 10 psi, so it didn't really tell me much. Where are you guys getting fuel pressure gauges that work with M20's? I called Autozone and Kragen and they only had the kind with a screw on fitting.

Matt

[nando]

so when you disconnected the TPS the hiccups went away?

5ml difference could just mean you have a dirty fuel filter. fuel pressure is 42.5psi so a 10psi gauge isn't going to tell you anything. I'd look for a pressure gauge from summit racing or harbor freight.

swap that TB and see what happens