rough running '90
Thread Starter
Burning Brakes
Joined: Nov 2002
Posts: 807
Likes: 21
From: Los Altos CA
rough running '90
I have developed a hard to solve rough running problem. There are no codes but it idles poorly and sometimes is hard to start when hot. Always starts quickly when cold and is smooth until it goes into closed loop. It seems to have reasonable power at full throttle. If I reset the computer and disconnect the oxygen sensor, it runs fairly well but not as well as it used to.
I have checked or replaced the following -IAC motor, MAP sensor, TPS, coolant temperature, MAT, oxygen sensor, plugs, wires, cap, rotor, ignition module, knock sensor, egr valve. I also changed the computer to another used one. I have done an injector balance test. Nothing seems to change the problem. The compression is ok on all cylinders. The AIR flow to the manifolds shuts off when it is supposed to.
Block learn ends up at 108 which is low. It as if the car wants to run rich and the oxygen sensor then leans it out until it is so lean it barely runs. It's not like there is a great vacuum leak - the IAC seems to control the idle speed ok.
Anyone have a problem like this and if so, what is the solution.
I have checked or replaced the following -IAC motor, MAP sensor, TPS, coolant temperature, MAT, oxygen sensor, plugs, wires, cap, rotor, ignition module, knock sensor, egr valve. I also changed the computer to another used one. I have done an injector balance test. Nothing seems to change the problem. The compression is ok on all cylinders. The AIR flow to the manifolds shuts off when it is supposed to.
Block learn ends up at 108 which is low. It as if the car wants to run rich and the oxygen sensor then leans it out until it is so lean it barely runs. It's not like there is a great vacuum leak - the IAC seems to control the idle speed ok.
Anyone have a problem like this and if so, what is the solution.
Melting Slicks
Joined: May 2004
Posts: 2,524
Likes: 10
From: Lufkin Texas
St. Jude Donor '05-'06
Try getting a stethescope and removing the cup (that doctor puts on chest) and put tube tip on injectors one at a time and listen to your injectors for normal ticking. I had an injector that was stopping every so often and when the stick open it floods cylinder and causes a rich condition. But, this should throw a code.
In light of everything you've already checked I must ask, have you checked fuel pressure??
Also, have you cleaned your T.B.? A dirty one will cause a hard to start when hot condition.
Just my 2 cents. Good luck.
In light of everything you've already checked I must ask, have you checked fuel pressure??
Also, have you cleaned your T.B.? A dirty one will cause a hard to start when hot condition.
Just my 2 cents. Good luck.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Check the resistance of your injectors when the engine is cold and when hot. You should have 16 to 17 ohms resistance for each injector. Any values lower than 16 should be suspected for shorted coils within the injector. Hope this is helpful!
Melting Slicks
Joined: May 2004
Posts: 2,524
Likes: 10
From: Lufkin Texas
St. Jude Donor '05-'06
Originally Posted by Sam Lam
Check the resistance of your injectors when the engine is cold and when hot. You should have 16 to 17 ohms resistance for each injector. Any values lower than 16 should be suspected for shorted coils within the injector. Hope this is helpful!
You also should check to see if injectors leak by checking if fuel pressure holds well after engine is off for a while.So, your TB is clean..hmmm. Thats good. This should go without asking but you didn't mention so I have to ask. Did you replace fuel filter??
Well, I'm out of suggestions, but here's a thread bump anyways.
Oh yeah, check that egr like 65ZO1 suggests and I meant to suggest a timing check myself, Glad Jim mentioned the timing light.
Last edited by skateparkdave; Jul 24, 2004 at 01:23 PM.
Thread Starter
Burning Brakes
Joined: Nov 2002
Posts: 807
Likes: 21
From: Los Altos CA
As suggeste, I did check the pressure with the engine shut off. It does drop - on the order of 10 psi in 1/2 hour. Of course there are other leakage paths - the regulator and back through the fuel pump.
I then kept the pressure up for an hour, disconnected all the injector electrical connections and cranked it. I could smell gasoline in the exhaust tips.
Then I dismantled the plenum and pulled the left hand injectors up and applied fuel pressure. In a half hour or so, the tips of all the injectors were wet with fuel although not enough to produce a drip. Is this enough to make the left bank excessively rich?
I then kept the pressure up for an hour, disconnected all the injector electrical connections and cranked it. I could smell gasoline in the exhaust tips.
Then I dismantled the plenum and pulled the left hand injectors up and applied fuel pressure. In a half hour or so, the tips of all the injectors were wet with fuel although not enough to produce a drip. Is this enough to make the left bank excessively rich?
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Melting Slicks
Joined: May 2004
Posts: 2,524
Likes: 10
From: Lufkin Texas
St. Jude Donor '05-'06
Hate to be the bringer of bad news, but it may be time for injector rebuild or new ones. 10psi pressure drop in a 1/2 hour is too much. The wet tips are a telltale sign. The fuel leaking is also is evaporating when injectors are out in the air, but when injectors are installed that fuel is just building up and sitting there in cylinder head causing hard warm start ups and reaking other havoc.
madvet.com has an Accel 24lbs/hr. set of 8 on sale for $299.00 and its what I got and am pleased so far. Ford SVO's are good also.
You can visit 65ZO1's site for links to injector replacements and solutions at:
http://www.geocities.com/jgkov/index.html
Good luck!
madvet.com has an Accel 24lbs/hr. set of 8 on sale for $299.00 and its what I got and am pleased so far. Ford SVO's are good also.
You can visit 65ZO1's site for links to injector replacements and solutions at:
http://www.geocities.com/jgkov/index.html
Good luck!
Last edited by skateparkdave; Jul 25, 2004 at 01:42 AM.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
I don't think either the leakdown or the observed leakage indicates a leaky injector issue.
If there is a misfire in the left bank the ECM will see (through the O2 sensor) excessive fuel and lean out the trim; a misfire in the right bank should have no effect on fuel trim, right. Actually, now I'm thinking a leaking EGR valve would make the mixture too lean so you would be seeing BLM values above 128.
BTW, was it at idle that you see the BLM value of 108; what was the Integrator value?
If there is a misfire in the left bank the ECM will see (through the O2 sensor) excessive fuel and lean out the trim; a misfire in the right bank should have no effect on fuel trim, right. Actually, now I'm thinking a leaking EGR valve would make the mixture too lean so you would be seeing BLM values above 128.
BTW, was it at idle that you see the BLM value of 108; what was the Integrator value?
Melting Slicks
Joined: May 2004
Posts: 2,524
Likes: 10
From: Lufkin Texas
St. Jude Donor '05-'06
Originally Posted by 65Z01
I don't think either the leakdown or the observed leakage indicates a leaky injector issue.
If there is a misfire in the left bank the ECM will see (through the O2 sensor) excessive fuel and lean out the trim; a misfire in the right bank should have no effect on fuel trim, right. Actually, now I'm thinking a leaking EGR valve would make the mixture too lean so you would be seeing BLM values above 128.
BTW, was it at idle that you see the BLM value of 108; what was the Integrator value?
If there is a misfire in the left bank the ECM will see (through the O2 sensor) excessive fuel and lean out the trim; a misfire in the right bank should have no effect on fuel trim, right. Actually, now I'm thinking a leaking EGR valve would make the mixture too lean so you would be seeing BLM values above 128.
BTW, was it at idle that you see the BLM value of 108; what was the Integrator value?
Well, Jims the expert. Better listen to him
My bad, I'll stand down.
Thread Starter
Burning Brakes
Joined: Nov 2002
Posts: 807
Likes: 21
From: Los Altos CA
Responding to 65Z01.
Wouldn't a misfire cause the ECM to think the system is lean? While there will be too much fuel in the exhaust there would be too much oxygen and the sensor measures oxygen.
A stuck open EGR valve would admit exhaust gases with little oxygen into the manifold. If this exhaust displaces fresh air and the fuel flow was the same, shouldn't itmake the system appear rich.
Please correct my logic if it is in error.
Wouldn't a misfire cause the ECM to think the system is lean? While there will be too much fuel in the exhaust there would be too much oxygen and the sensor measures oxygen.
A stuck open EGR valve would admit exhaust gases with little oxygen into the manifold. If this exhaust displaces fresh air and the fuel flow was the same, shouldn't itmake the system appear rich.
Please correct my logic if it is in error.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Well, though that leak down rate could indicate some injector leakage (or another issue) I don't think it is the cause of his problem (IMHO, I'm certainly not an expert here).
I may be wrong about the stuck EGR valve, to test for that condition, apply vacuum directly to the EGR valve at idle and observe the BLM value & Int changes.
Similarly, to check of the effect of a misfire, pull a plug wire on the left bank and observe those changes.
Right now I'm messing with a dead cruise control; be back in a short.
I may be wrong about the stuck EGR valve, to test for that condition, apply vacuum directly to the EGR valve at idle and observe the BLM value & Int changes.
Similarly, to check of the effect of a misfire, pull a plug wire on the left bank and observe those changes.
Right now I'm messing with a dead cruise control; be back in a short.
